xref: /aosp_15_r20/external/pytorch/torch/distributed/fsdp/_init_utils.py (revision da0073e96a02ea20f0ac840b70461e3646d07c45)

# mypy: allow-untyped-defs
import collections
import itertools
import os
import warnings
from typing import (
    Any,
    Callable,
    Deque,
    Dict,
    Generator,
    Iterable,
    Iterator,
    List,
    no_type_check,
    Optional,
    Set,
    Tuple,
    TYPE_CHECKING,
    Union,
)

import torch
import torch.distributed as dist
import torch.distributed.fsdp._exec_order_utils as exec_order_utils
import torch.distributed.fsdp._traversal_utils as traversal_utils
import torch.distributed.fsdp.fully_sharded_data_parallel as fsdp_file
import torch.nn as nn
from torch.distributed.algorithms._comm_hooks import default_hooks
from torch.distributed.device_mesh import _mesh_resources, DeviceMesh
from torch.distributed.distributed_c10d import _get_default_group
from torch.distributed.fsdp._common_utils import (
    _FSDPDeviceHandle,
    _FSDPState,
    _get_module_fsdp_state,
    _is_fsdp_flattened,
    _named_parameters_with_duplicates,
    clean_tensor_name,
    TrainingState,
)
from torch.distributed.fsdp._flat_param import (
    _FSDP_USE_FULL_PREC_IN_EVAL,
    FlatParameter,
    FlatParamHandle,
    HandleShardingStrategy,
)
from torch.distributed.fsdp._limiter_utils import _FreeEventQueue
from torch.distributed.fsdp.api import (
    BackwardPrefetch,
    CPUOffload,
    FullOptimStateDictConfig,
    FullStateDictConfig,
    MixedPrecision,
    ShardingStrategy,
    StateDictConfig,
    StateDictType,
)
from torch.distributed.fsdp.wrap import _Policy
from torch.distributed.tensor.parallel.fsdp import DTensorExtensions
from torch.distributed.utils import _sync_params_and_buffers
from torch.utils._python_dispatch import is_traceable_wrapper_subclass


if TYPE_CHECKING:
    from torch.utils.hooks import RemovableHandle

_TORCHDISTX_AVAIL = True
try:
    from torchdistx import deferred_init, fake  # type: ignore[import]
except ImportError:
    _TORCHDISTX_AVAIL = False

PARAM_BROADCAST_BUCKET_SIZE = int(250 * 1024 * 1024)
FSDP_SYNCED = "_fsdp_synced"
# Specification of process groups for hybrid sharding strategies.
HybridShardProcessGroupType = Tuple[dist.ProcessGroup, dist.ProcessGroup]
# Overall specification of process group.
ProcessGroupType = Optional[Union[dist.ProcessGroup, HybridShardProcessGroupType]]


# TODO (awgu): Refactor this later
SHARDING_STRATEGY_MAP = {
    ShardingStrategy.NO_SHARD: HandleShardingStrategy.NO_SHARD,
    ShardingStrategy.FULL_SHARD: HandleShardingStrategy.FULL_SHARD,
    ShardingStrategy.SHARD_GRAD_OP: HandleShardingStrategy.SHARD_GRAD_OP,
    ShardingStrategy.HYBRID_SHARD: HandleShardingStrategy.HYBRID_SHARD,
    ShardingStrategy._HYBRID_SHARD_ZERO2: HandleShardingStrategy._HYBRID_SHARD_ZERO2,
}
HYBRID_SHARDING_STRATEGIES = [
    ShardingStrategy.HYBRID_SHARD,
    ShardingStrategy._HYBRID_SHARD_ZERO2,
]
NO_RESHARD_AFTER_FORWARD_STRATEGIES = (
    ShardingStrategy.SHARD_GRAD_OP,
    ShardingStrategy._HYBRID_SHARD_ZERO2,
)


# NOTE: Since non-self attributes cannot be type annotated, several attributes
# on `state` are defined first as local variables before being assigned.


@no_type_check
def _init_process_group_state(
    state: _FSDPState,
    process_group: ProcessGroupType,
    sharding_strategy: ShardingStrategy,
    policy: Optional[_Policy],
    device_mesh: Optional[DeviceMesh] = None,
) -> _FSDPState:
    if process_group is not None and device_mesh is not None:
        raise ValueError(
            "Cannot pass both process_group and device_mesh at the "
            "same time. Please just pass only one of them."
        )
    is_hybrid_strategy = sharding_strategy in HYBRID_SHARDING_STRATEGIES
    if is_hybrid_strategy:
        if process_group is None and policy is None and device_mesh is None:
            # Raise an error here, since this is manual wrapping with no process group
            # passed in, there is no way to ensure all wrapped FSDP instances use the same
            # process groups.
            raise ValueError(
                f"Manual wrapping with {sharding_strategy} "
                "requires explicit specification of process group or device_mesh."
            )
        else:
            state = _init_process_group_state_for_hybrid_shard(
                state, process_group, device_mesh
            )
    else:
        if device_mesh:
            state._device_mesh = device_mesh
            state.process_group = device_mesh.get_group(mesh_dim=0)
        else:
            state.process_group = (
                process_group if process_group is not None else _get_default_group()
            )

    state.rank = state.process_group.rank()
    state.world_size = state.process_group.size()
    data_parallel_world_size = state.world_size
    if is_hybrid_strategy:
        data_parallel_world_size *= state._inter_node_pg.size()
    state._gradient_predivide_factor = (
        default_hooks.DefaultState._get_gradient_predivide_factor(
            data_parallel_world_size
        )
    )
    state._gradient_postdivide_factor = (
        data_parallel_world_size / state._gradient_predivide_factor
    )
    return state


@no_type_check
def _init_process_group_state_for_hybrid_shard(
    state: _FSDPState,
    process_group: ProcessGroupType,
    device_mesh: DeviceMesh,
) -> _FSDPState:
    if device_mesh:
        if _is_valid_hybrid_shard_device_mesh(device_mesh):
            state._device_mesh = device_mesh
            # We currently only allow _inter_node_pg to be the outermost dimension, and the
            # process_group(intra_node) to be the innermost dimension.
            state._inter_node_pg = device_mesh.get_group(mesh_dim=0)
            state.process_group = device_mesh.get_group(mesh_dim=1)
        else:
            raise ValueError(
                f"Expected device_mesh to have ndim=2 but got {device_mesh.ndim}"
            )
    elif process_group is None:
        default_group = _get_default_group()
        intra_node_group, inter_node_group = _init_intra_and_inter_node_groups(
            default_group, state._device_handle.device_count()
        )
        # we shard across intra-node
        state.process_group = intra_node_group
        # save _inter_node_pg to allreduce across.
        state._inter_node_pg = inter_node_group
    else:
        # Check type and assign state.process_group and state._inter_node_pg.
        if _is_valid_hybrid_shard_pg_type(process_group):
            # Assuming that user passed in as intra node group and inter node group
            # as documented.
            state.process_group, state._inter_node_pg = process_group
        else:
            raise ValueError(
                "Expected process_group to be passed in as either None or "
                f"Tuple[dist.ProcessGroup, dist.ProcessGroup] but got {type(process_group)}"
            )
    # Create state for allreduce
    state._inter_node_state = _get_default_comm_hook_state(
        process_group=state._inter_node_pg,
    )
    return state


@no_type_check
def _is_valid_hybrid_shard_pg_type(process_group: Any) -> bool:
    return (
        isinstance(process_group, tuple)
        and len(process_group) == 2
        and all(isinstance(pg, dist.ProcessGroup) for pg in process_group)
    )


@no_type_check
def _is_valid_hybrid_shard_device_mesh(device_mesh: DeviceMesh) -> bool:
    return isinstance(device_mesh, DeviceMesh) and device_mesh.ndim == 2


@no_type_check
def _init_intra_node_process_group(num_devices_per_node: int) -> dist.ProcessGroup:
    """
    Return a process group across the current node.

    For example, given each row is a distinct node:
    0  1  2  3  4  5  6  7
    8  9 10 11 12 13 14 15
    This API would return an intra-node subgroup across
    [0, 1, ..., 7] or [8, 9, ..., 15] depending on the process's rank.
    For example, rank 3 would get [0, 1, ..., 7].
    """
    intra_node_subgroup, _ = dist.new_subgroups(num_devices_per_node)
    return intra_node_subgroup


@no_type_check
def _init_inter_node_process_group(
    global_process_group: dist.ProcessGroup,
    num_devices_per_node: int,
) -> dist.ProcessGroup:
    """
    Return an inter-node process group where each contained rank has the same local rank.

    For example, given each row is a distinct node:
    0  1  2  3  4  5  6  7
    8  9 10 11 12 13 14 15
    This API would return inter-node process group [0, 8], [1, 9], [2, 10], and so forth
    depending on the process's rank. For example, rank 1 would get [1, 9], rank 5
    would get [5, 13].
    """
    # the inter-node pg that is returned
    inter_node_pg = None
    sharding_backend = dist.get_backend(global_process_group)
    world_size = dist.get_world_size(global_process_group)
    # Assuming fully homogeneous setup
    num_nodes = world_size // num_devices_per_node
    my_local_rank = dist.get_rank(global_process_group) % num_devices_per_node
    for local_rank in range(num_devices_per_node):
        ranks_for_inter_group = [
            local_rank + (i * num_devices_per_node) for i in range(num_nodes)
        ]
        # every rank always needs to call dist.new_group
        grp = dist.new_group(ranks=ranks_for_inter_group, backend=sharding_backend)
        if local_rank == my_local_rank:
            inter_node_pg = grp

    assert (
        inter_node_pg is not None
    ), f"{my_local_rank} expected to assign inter-node pg, but did not"
    return inter_node_pg


def _init_intra_and_inter_node_groups(
    global_process_group: dist.ProcessGroup,
    num_devices_per_node: int,
) -> Tuple[dist.ProcessGroup, dist.ProcessGroup]:
    """
    Initialize intra and inter-node process groups and return the ones corresponding to this process's rank.

    This function can be used to initialize process groups for ``HYBRID_SHARD`` or
    ``_HYBRID_SHARD_ZERO2`` in FSDP.
    This function assumes each node has an equal number of CUDA-enabled devices.
    Returns:
        Tuple[dist.ProcessGroup, dist.ProcessGroup]: Intra and inter-node process group.
    """
    return (
        _init_intra_node_process_group(num_devices_per_node),
        _init_inter_node_process_group(global_process_group, num_devices_per_node),
    )


@no_type_check
def _init_ignored_module_states(
    state: _FSDPState,
    module: nn.Module,
    ignored_modules: Optional[Iterable[torch.nn.Module]],
    ignored_states: Union[
        Optional[Iterable[torch.nn.Parameter]], Optional[Iterable[torch.nn.Module]]
    ] = None,
) -> _FSDPState:
    if ignored_modules is not None and ignored_states is not None:
        raise ValueError(
            "Cannot pass both ignored_modules and ignored_states at the "
            "same time. Please just pass ignored_states."
        )
    ignored_parameters = None
    passed_as_ignored_states = ignored_states is not None
    if passed_as_ignored_states:
        ignored_states_list = list(ignored_states)
        _check_ignored_states(ignored_states_list, True)
    else:
        ignored_states_list = []
        _check_ignored_states(
            list(ignored_modules) if ignored_modules is not None else [], False
        )
    if len(ignored_states_list) > 0:
        if isinstance(ignored_states_list[0], nn.Parameter):
            ignored_parameters = ignored_states_list
        else:
            ignored_modules = ignored_states_list
    state._ignored_modules = _get_ignored_modules(module, ignored_modules)
    state._ignored_params = _get_ignored_params(
        module,
        state._ignored_modules,
        ignored_parameters,
    )
    state._ignored_buffer_names = _get_ignored_buffer_names(
        module,
        state._ignored_modules,
    )
    # TODO: FSDP's contract for buffers is not well-defined. They are
    # implicitly ignored for most functionality since they are not sharded;
    # however, FSDP still imposes some semantics on buffers (e.g. buffer mixed
    # precision). We should formalize this contract and decide if we need to
    # compute and store `_ignored_buffers`.
    return state


def _check_ignored_states(
    ignored_states: List[Any], passed_as_ignored_states: bool
) -> None:
    """
    Check that the ignored states are uniformly parameters or uniformly modules.

    We may remove this check in the future if we permit mixing.
    """
    if len(ignored_states) == 0:
        return
    if passed_as_ignored_states:
        all_params = all(isinstance(state, nn.Parameter) for state in ignored_states)
        all_modules = all(isinstance(state, nn.Module) for state in ignored_states)
        if not all_params and not all_modules:
            # Sort for consistent ordering for unit test regex matching
            sorted_types = sorted({type(state) for state in ignored_states}, key=repr)
            raise ValueError(
                "ignored_states expects all nn.Parameter or all nn.Module list "
                f"elements but got types {sorted_types}"
            )
    else:
        if not all(isinstance(state, nn.Module) for state in ignored_states):
            sorted_types = sorted({type(state) for state in ignored_states}, key=repr)
            raise ValueError(
                "ignored_modules expects nn.Module list elements but got "
                f"types {sorted_types}"
            )


@no_type_check
def _init_device_handle(
    state: _FSDPState,
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
    device_id: Optional[Union[int, torch.device]],
) -> _FSDPState:
    """
    Determine device handle used for initializing FSDP.

    If a device is specified by ``device_id``,
    then returns device handle corresponds to that device type. Otherwise, If the
    module is already on a non-CPU device, then the device type is that non-CPU device type.
    If the module is on CPU or meta, then the device type is the current accelerator device.
    See the :ref:`Accelerators<accelerators>` for details.


    This method will be called once ignored paramters was determined, as the device handle maybe needed
    for other initialization.
    """
    determined_device = None
    if device_id is not None:
        determined_device = (
            device_id
            if isinstance(device_id, torch.device)
            else torch.device(device_id)
        )
    if determined_device is None:
        for param in _get_orig_params(module, ignored_params):
            if param.device.type in {"cpu", "meta"}:
                continue
            if determined_device is None:
                determined_device = param.device
            else:
                if param.device.type != determined_device.type:
                    raise RuntimeError(
                        f"FSDP does not support modules with different device types "
                        f"but got params on {determined_device.type} and {param.device.type}"
                    )
        determined_device = determined_device or torch._C._get_accelerator()
        if determined_device.type == "cpu":
            raise RuntimeError(
                "FSDP needs a non-CPU accelerator device, but no accelerator device is detected."
            )

    state._device_handle = _FSDPDeviceHandle.from_device(determined_device)
    return state


@no_type_check
def _init_buffer_state(
    state: _FSDPState,
    module: nn.Module,
) -> _FSDPState:
    state._buffer_names = _get_buffer_names(module)
    # Save a mapping from clean fully-qualified buffer name (starting from
    # `module`) to its original dtype for restoring that dtype during model
    # checkpointing when buffer mixed precision is enabled. The names should
    # be clean since the casting happens in a `summon_full_params()` context.
    _buffer_name_to_orig_dtype: Dict[str, torch.dtype] = {}
    for buffer_name, buffer in module.named_buffers():
        buffer_name = clean_tensor_name(buffer_name)
        _buffer_name_to_orig_dtype[buffer_name] = buffer.dtype
    state._buffer_name_to_orig_dtype = _buffer_name_to_orig_dtype
    return state


@no_type_check
def _init_core_state(
    state: _FSDPState,
    sharding_strategy: Optional[ShardingStrategy],
    mixed_precision: Optional[MixedPrecision],
    cpu_offload: Optional[CPUOffload],
    limit_all_gathers: bool,
    use_orig_params: bool,
    backward_prefetch_limit: int,
    forward_prefetch_limit: int,
) -> _FSDPState:
    # We clamp the strategy to `NO_SHARD` for world size of 1 since they are
    # currently functionally equivalent. This may change if/when we integrate
    # FSDP with MoE.
    if state.world_size == 1:
        if sharding_strategy != ShardingStrategy.NO_SHARD:
            warnings.warn(
                "FSDP is switching to use `NO_SHARD` instead of "
                f"{sharding_strategy or ShardingStrategy.FULL_SHARD} since "
                "the world size is 1."
            )
        sharding_strategy = ShardingStrategy.NO_SHARD
    elif sharding_strategy == ShardingStrategy.NO_SHARD:
        warnings.warn(
            "The `NO_SHARD` sharding strategy is deprecated. If having issues, "
            "please use `DistributedDataParallel` instead.",
            FutureWarning,
            # Level 1 is here, level 2 is from `FullyShardedDataParallel`, and
            # level 3 is from the true caller
            stacklevel=3,
        )
    state.sharding_strategy = sharding_strategy or ShardingStrategy.FULL_SHARD
    state.mixed_precision = mixed_precision or MixedPrecision()
    if mixed_precision is not None:
        torch._C._log_api_usage_once(
            f"torch.distributed.fsdp.mixed_precision.{str(state.mixed_precision)}"
        )
    state._use_full_prec_in_eval = (
        os.environ.get(_FSDP_USE_FULL_PREC_IN_EVAL, "") == "1"
    )
    state.cpu_offload = cpu_offload or CPUOffload()
    state.limit_all_gathers = limit_all_gathers
    state._use_orig_params = use_orig_params
    state.training_state = TrainingState.IDLE
    state._is_root = None
    state._free_event_queue = _FreeEventQueue()
    state._debug_level = dist.get_debug_level()
    state._exec_order_data = exec_order_utils._ExecOrderData(
        state._debug_level,
        backward_prefetch_limit,
        forward_prefetch_limit,
    )
    state._unshard_event = None
    # Mapping from fully sharded module to the handles it is responsible to
    # unshard and reshard (see [Note: Fully Sharded Module])
    _fully_sharded_module_to_handle: Dict[nn.Module, FlatParamHandle] = {}
    state._fully_sharded_module_to_handle = _fully_sharded_module_to_handle
    # Invariant: `state.params` contains exactly the `FlatParameter`s of the
    # handles in `state._handle`
    _handle: Optional[FlatParamHandle] = None
    state._handle = _handle
    params: List[FlatParameter] = []
    state.params = params
    return state


@no_type_check
def _init_runtime_state(
    state: _FSDPState,
) -> _FSDPState:
    _root_pre_forward_handles: List[RemovableHandle] = []
    state._root_pre_forward_handles = _root_pre_forward_handles
    _pre_forward_handles: List[RemovableHandle] = []
    state._pre_forward_handles = _pre_forward_handles
    _post_forward_handles: List[RemovableHandle] = []
    state._post_forward_handles = _post_forward_handles
    state._sync_gradients = True
    state._comm_hook = None
    state._comm_hook_state = None
    # Used to prevent running the pre-backward hook multiple times
    return state


@no_type_check
def _init_prefetching_state(
    state: _FSDPState,
    backward_prefetch: BackwardPrefetch,
    forward_prefetch: bool,
) -> _FSDPState:
    state.backward_prefetch = backward_prefetch
    state.forward_prefetch = forward_prefetch
    # The data structures use tuples of handles to generalize over the case
    # where a module's forward involves multiple handles.
    return state


@no_type_check
def _init_extension(state: _FSDPState, device_mesh: DeviceMesh = None) -> _FSDPState:
    # TODO: we need to add additional check once we support FSDP + PiPPy.
    # This check is currently sufficient, since we only support FSDP + TP.
    root_mesh = _mesh_resources.get_root_mesh(device_mesh)
    # if a root mesh is not the same as device_mesh,
    # meaning the device_mesh is sliced out from the root mesh.
    if device_mesh and root_mesh != state._device_mesh:
        state._fsdp_extension = DTensorExtensions(state._device_handle)
    else:
        # We need to explicilty set _fsdp_extension to None.
        # Otherwise, we will run into an infinite recursion when getting the attribute.
        state._fsdp_extension = None
    return state


@no_type_check
def _init_state_dict_state(state: _FSDPState) -> _FSDPState:
    state._state_dict_type = StateDictType.FULL_STATE_DICT
    state_dict_config: StateDictConfig = FullStateDictConfig()
    state._optim_state_dict_config = FullOptimStateDictConfig()
    state._state_dict_config = state_dict_config
    unshard_params_ctx: Dict[nn.Module, Generator] = {}
    state._unshard_params_ctx = unshard_params_ctx

    return state


def _verify_managed_params(module: nn.Module, params: List[nn.Parameter]) -> None:
    """
    Verify if the parameters are accepted by FSDP. The only restriction now
    is that the parameter cannot be a scalar tensor (param.shape == []).
    """
    for param in params:
        if len(param.shape) == 0:
            param_name = ""
            for name, param_ in module.named_parameters():
                if param is param_:
                    param_name = name
                    break
            assert param_name
            raise ValueError(
                "FSDP doesn't support salar parameters. "
                f"Change {param_name} to a 1D tensor with numel equal to 1."
            )


@no_type_check
def _init_param_handle_from_module(
    state: _FSDPState,
    fully_sharded_module: nn.Module,
    device_id: Optional[Union[int, torch.device]],
    param_init_fn: Optional[Callable[[nn.Module], None]],
    sync_module_states: bool,
) -> _FSDPState:
    """Initialize a ``FlatParamHandle`` from a module ``fully_sharded_module``."""
    _check_single_device_module(fully_sharded_module, state._ignored_params, device_id)
    device_from_device_id = _get_device_from_device_id(
        device_id, state.rank, state._device_handle
    )
    is_meta_module, is_torchdistX_deferred_init = _need_to_materialize_module(
        fully_sharded_module, state._ignored_params, state._ignored_modules
    )
    # Materialize the module if needed
    if (is_meta_module or is_torchdistX_deferred_init) and param_init_fn is not None:
        _materialize_with_param_init_fn(
            fully_sharded_module, param_init_fn, state._ignored_modules
        )
    elif is_meta_module:
        _materialize_meta_module(
            fully_sharded_module,
            device_id,
            state._ignored_modules,
            state._device_handle,
        )
    elif is_torchdistX_deferred_init:
        deferred_init.materialize_module(
            fully_sharded_module,
            check_fn=lambda submodule: _get_module_fsdp_state(submodule) is None
            and submodule not in state._ignored_modules,
        )

    ignored_buffers = {
        buffer
        for ignored_module in state._ignored_modules
        for buffer in ignored_module.buffers()
    }

    _move_module_to_device(
        fully_sharded_module,
        state._ignored_params,
        ignored_buffers,
        device_from_device_id,
    )
    state.compute_device = _get_compute_device(
        fully_sharded_module,
        state._ignored_params,
        device_from_device_id,
        state.rank,
        state._device_handle,
    )

    managed_params = list(_get_orig_params(fully_sharded_module, state._ignored_params))
    _verify_managed_params(fully_sharded_module, managed_params)
    if sync_module_states:
        _sync_module_params_and_buffers(
            fully_sharded_module, managed_params, state.process_group
        )
        if state.sharding_strategy in HYBRID_SHARDING_STRATEGIES:
            _sync_module_params_and_buffers(
                fully_sharded_module, managed_params, state._inter_node_pg
            )
    _init_param_handle_from_params(state, managed_params, fully_sharded_module)
    return state


@no_type_check
def _init_param_handle_from_params(
    state: _FSDPState,
    params: List[nn.Parameter],
    fully_sharded_module: nn.Module,
):
    if len(params) == 0:
        return
    handle = FlatParamHandle(
        params,
        fully_sharded_module,
        state.compute_device,
        SHARDING_STRATEGY_MAP[state.sharding_strategy],
        state.cpu_offload.offload_params,
        state.mixed_precision.param_dtype,
        state.mixed_precision.reduce_dtype,
        state.mixed_precision.keep_low_precision_grads,
        state.process_group,
        state._use_orig_params,
        fsdp_extension=state._fsdp_extension,
    )
    handle.shard()
    assert not state._handle
    state.params.append(handle.flat_param)
    state._handle = handle
    state._fully_sharded_module_to_handle[handle._fully_sharded_module] = handle
    cpu_device = torch.device("cpu")
    if state.cpu_offload.offload_params and handle.flat_param.device != cpu_device:
        handle.flat_param_to(cpu_device)


def _get_ignored_modules(
    root_module: nn.Module,
    _ignored_modules: Optional[Iterable[torch.nn.Module]],
) -> Set[nn.Module]:
    """
    Check that ``_ignored_modules`` is an iterable of ``nn.Module`` s without any FSDP instances.

    Return the modules contained in their module
    subtrees as a :class:`set`. Nested FSDP instances are excluded, but their
    already-computed ignored modules are included.

    ``_ignored_modules`` represents the argument passed by the user to FSDP.
    """
    msg_prefix = "`ignored_modules` should be an iterable of `torch.nn.Module`s "
    try:
        ignored_root_modules = (
            set(_ignored_modules) if _ignored_modules is not None else set()
        )
    except TypeError as e:
        raise TypeError(msg_prefix + f"but got {type(_ignored_modules)}") from e
    for module in ignored_root_modules:
        if not isinstance(module, torch.nn.Module):
            raise TypeError(msg_prefix + f"but got an iterable with {type(module)}")
        if _get_module_fsdp_state(module):
            # TODO: We may relax this by taking the FSDP instance's wrapped
            # module to provide more flexibility to the user.
            raise ValueError("`ignored_modules` should not include FSDP modules")
    # Treat modules that cannot compose with `fully_shard` as ignored modules,
    # meaning that their subtrees are ignored
    for module in root_module.modules():
        if not traversal_utils._composable(module):
            ignored_root_modules.add(module)
    # NOTE: Even if `ignored_root_modules` is empty, do not return early so
    # that this FSDP instance can get any ignored modules from its children.

    # Include child modules and exclude nested FSDP modules themselves
    ignored_modules = {
        child
        for module in ignored_root_modules
        for child in module.modules()
        if not isinstance(child, fsdp_file.FullyShardedDataParallel)
    }
    if root_module in ignored_modules:
        warnings.warn(
            "Trying to ignore the top-level module passed into the FSDP "
            "constructor itself will result in all parameters being "
            f"ignored and is not well-supported: {module}"
        )
    # Include nested FSDP modules' ignored modules
    for submodule in root_module.modules():
        optional_fsdp_state = _get_module_fsdp_state(submodule)
        if optional_fsdp_state is not None:
            assert hasattr(optional_fsdp_state, "_ignored_modules")
            ignored_modules.update(optional_fsdp_state._ignored_modules)
    return ignored_modules


def _get_ignored_params(
    root_module: torch.nn.Module,
    ignored_modules: Set[torch.nn.Module],
    ignored_parameters: Optional[Iterable[torch.nn.Parameter]] = None,
) -> Set[torch.nn.Parameter]:
    """
    Return the parameters of the modules in ``ignored_modules`` and the parameters in ``ignored_parameters``.

    :class:`FlatParameter` s are excluded from the result.
    """
    all_ignored_params: Set[torch.nn.Parameter] = set()

    params_in_ignored_modules = {
        p for m in ignored_modules for p in m.parameters() if not _is_fsdp_flattened(p)
    }

    all_ignored_params.update(params_in_ignored_modules)

    if ignored_parameters is not None:
        params_in_ignored_parameters = {
            p for p in ignored_parameters if not _is_fsdp_flattened(p)
        }
        all_ignored_params.update(params_in_ignored_parameters)

    # Always include nested FSDP modules' ignored parameters
    for submodule in root_module.modules():
        optional_fsdp_state = _get_module_fsdp_state(submodule)
        if optional_fsdp_state is not None:
            assert hasattr(optional_fsdp_state, "_ignored_params")
            all_ignored_params.update(optional_fsdp_state._ignored_params)

    return all_ignored_params


def _get_ignored_buffer_names(
    root_module: torch.nn.Module,
    ignored_modules: Set[torch.nn.Module],
) -> Set[str]:
    """Return the cleaned buffer FQNs in ``ignored_modules``."""
    all_ignored_buffer_names: Set[str] = set()

    buffers_in_ignored_modules = {
        buffer for m in ignored_modules for buffer in m.buffers()
    }

    all_ignored_buffer_names.update(
        {
            clean_tensor_name(buffer_name)
            for buffer_name, buffer in root_module.named_buffers()
            if buffer in buffers_in_ignored_modules
        }
    )

    # Always include nested FSDP modules' ignored buffer names
    for submodule in root_module.modules():
        optional_fsdp_state = _get_module_fsdp_state(submodule)
        if optional_fsdp_state is not None:
            assert hasattr(optional_fsdp_state, "_ignored_buffer_names")
            all_ignored_buffer_names.update(optional_fsdp_state._ignored_buffer_names)

    return all_ignored_buffer_names


def _get_buffer_names(root_module: nn.Module) -> Set[str]:
    """Return the fully prefixed names of all buffers in the module hierarchy rooted at ``root_module`` as a class:`set`."""
    return {
        clean_tensor_name(buffer_name) for buffer_name, _ in root_module.named_buffers()
    }


def _check_single_device_module(
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
    device_id: Optional[Union[int, torch.device]],
) -> None:
    """
    Raise an error if ``module`` has original parameters on multiple devices, ignoring the parameters in ``ignored_params``.

    Thus, after this method, the
    module must be either fully on the CPU or fully on a non-CPU device.
    """
    devices = {param.device for param in _get_orig_params(module, ignored_params)}
    # We allow module to be partially on CPU and partially on GPU if device_id is not
    # None, since the device_id arg will result in the CPU portion being moved to
    # GPU. This is useful in cases where part of the module may be parallelized
    # by another algorithm and may already be on GPU. We'd like to enforce device_id
    # to not be None, otherwise we'd flatten parameters in a mixed module which is
    # not supported.
    if len(devices) == 2 and torch.device("cpu") in devices:
        if device_id is None:
            raise RuntimeError(
                "To support a module with both CPU and GPU params, "
                "please pass in device_id argument."
            )
    elif len(devices) > 1:
        raise RuntimeError(
            f"FSDP only supports single device modules but got params on {devices}"
        )


def _get_device_from_device_id(
    device_id: Optional[Union[int, torch.device]],
    rank: int,
    device_handle: _FSDPDeviceHandle,
) -> Optional[torch.device]:
    """
    Return a ``torch.device`` for the specified ``device_id``.

    Processes ``device_id`` and returns either the corresponding device or
    ``None`` if ``device_id`` is ``None``.
    """
    if device_id is None:
        return None
    device = (
        device_id if isinstance(device_id, torch.device) else torch.device(device_id)
    )
    if device.type != "cpu" and device.index is None:
        warnings.warn(
            f"FSDP got the argument `device_id` {device_id} on rank "
            f"{rank}, which does not have an explicit index. "
            f"FSDP will use the current device {device_handle.current_device()}. "
            f"If this is incorrect, please explicitly call `torch.{device.type}.set_device()` "
            "before FSDP initialization or pass in the explicit device "
            "index as the `device_id` argument."
        )
        device = torch.device(device_handle.current_device())
    return device


def _need_to_materialize_module(
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
    ignored_modules: Set[nn.Module],
) -> Tuple[bool, bool]:
    """
    Return if ``module`` has parameters on meta device and if ``module`` is using torchdistX deferred initialization.

    At most of the returned bools can
    be ``True``. If either is ``True``, then ``module`` needs to be
    materialized.
    """
    managed_params = list(_get_orig_params(module, ignored_params))
    is_meta_module = any(param.is_meta for param in managed_params)
    # TODO: We need to establish a contract for FSDP and buffers. For now, we
    # skip checking for meta buffers from ignored modules. We should consider
    # refactoring the initialization holistically to avoid so many traversals.
    for submodule in module.modules():
        if submodule in ignored_modules:
            continue
        for buf in submodule.buffers(recurse=False):
            is_meta_module |= buf.is_meta
    is_torchdistX_deferred_init = (
        not is_meta_module
        and _TORCHDISTX_AVAIL
        and any(fake.is_fake(param) for param in managed_params)
    )
    return is_meta_module, is_torchdistX_deferred_init


def _materialize_with_param_init_fn(
    root_module: nn.Module,
    param_init_fn: Callable[[nn.Module], None],
    ignored_modules: Set[nn.Module],
) -> None:
    if not callable(param_init_fn):
        raise ValueError(
            f"Expected {param_init_fn} to be callable but got {type(param_init_fn)}"
        )
    modules_to_materialize = _get_modules_to_materialize(root_module, ignored_modules)
    for module in modules_to_materialize:
        param_init_fn(module)


def _materialize_meta_module(
    root_module: nn.Module,
    device_from_device_id: Optional[torch.device],
    ignored_modules: Set[nn.Module],
    device_handle: _FSDPDeviceHandle,
):
    # Run default meta device initialization
    materialization_device = device_from_device_id or torch.device(
        device_handle.current_device()
    )
    modules_to_materialize = _get_modules_to_materialize(root_module, ignored_modules)
    module = None
    try:
        # Assume that each module's `reset_parameters()` only initializes its
        # own parameters and not those of its children
        with torch.no_grad():
            for module in modules_to_materialize:
                # As a contract to the user, only call `reset_parameters()` if
                # the module has directly managed parameters/buffers
                module_state_iter = itertools.chain(
                    module.parameters(recurse=False), module.buffers(recurse=False)
                )
                has_module_states = len(list(module_state_iter)) > 0
                if has_module_states:
                    module.to_empty(device=materialization_device, recurse=False)
                    module.reset_parameters()  # type: ignore[operator]
    except BaseException as e:
        warnings.warn(
            "Unable to call `reset_parameters()` for module on meta "
            f"device with error {str(e)}. Please ensure that your module of"
            f"type {type(module)} implements a `reset_parameters()` method."  # type: ignore[possibly-undefined]
        )
        raise e


def _get_modules_to_materialize(
    root_module: nn.Module, ignored_modules: Set[nn.Module]
) -> List[nn.Module]:
    # Run BFS to collect the modules to materialize via `reset_parameters()`,
    # stopping at any module with FSDP already applied or at ignored modules.
    modules_to_materialize: List[nn.Module] = []
    queue = collections.deque([root_module])
    visited_modules: Set[nn.Module] = {root_module}
    while queue:
        module = queue.popleft()
        modules_to_materialize.append(module)
        for child_module in module.children():
            if (
                child_module not in visited_modules
                and _get_module_fsdp_state(child_module) is None
                and child_module not in ignored_modules
            ):
                visited_modules.add(child_module)
                queue.append(child_module)
    return modules_to_materialize


def _move_module_to_device(
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
    ignored_buffers: Set[torch.Tensor],
    device_from_device_id: Optional[torch.device],
) -> None:
    """
    Move ``module`` depending on ``device_from_device_id`` and its current device.

    This includes moving ignored modules' parameters.

    - If ``device_from_device_id`` is not ``None``, then this moves
    ``module`` to the device.
    - If ``device_from_device_id`` is ``None``, then this does not move
    ``module`` but warns the user if it is on CPU.

    Precondition: ``_check_single_device_module()``.
    """
    cpu_device = torch.device("cpu")
    if device_from_device_id is not None:
        # BFS from `module` without traversing any nested FSDP instances to
        # collect the parameters/buffers that have not yet been managed
        queue: Deque[nn.Module] = collections.deque()
        queue.append(module)
        params: List[nn.Parameter] = []
        buffers: List[torch.Tensor] = []
        while queue:
            curr_module = queue.popleft()
            # NOTE: We include a check to only move parameters/buffers that are
            # on CPU device. If they are on a CUDA device different from the
            # one specified by `device_id`, then this does NOT move them. This
            # is so that we can raise an error in `_get_compute_device()`.
            params.extend(
                param
                for param in curr_module.parameters(recurse=False)
                if param.device == cpu_device
            )
            buffers.extend(
                buffer
                for buffer in curr_module.buffers(recurse=False)
                if buffer.device == cpu_device
            )
            for submodule in curr_module.children():
                if not isinstance(submodule, fsdp_file.FullyShardedDataParallel):
                    queue.append(submodule)
        params_to_move = [p for p in params if p not in ignored_params]
        bufs_to_move = [p for p in buffers if p not in ignored_buffers]
        _move_states_to_device(params_to_move, bufs_to_move, device_from_device_id)
        return
    param = next(_get_orig_params(module, ignored_params), None)
    if param is not None and param.device == cpu_device:
        _warn_cpu_init()


def _move_states_to_device(
    params: List[nn.Parameter],
    buffers: List[torch.Tensor],
    device_from_device_id: Optional[torch.device],
) -> None:
    """
    Move states to the specified device.

    Precondition: ``_check_single_device_module()`` and module's parameters and
    buffers have been materialized if needed.
    """
    if len(params) == 0 and len(buffers) == 0:
        return
    if len(params) > 0:
        current_device = params[0].device
    elif len(buffers) > 0:
        current_device = buffers[0].device
    cpu_device = torch.device("cpu")
    if device_from_device_id is not None:
        # Move the parameters and buffers like the `.data` code path in
        # `nn.Module._apply()`, which underlies `nn.Module.to()`
        for param in params:
            with torch.no_grad():
                param.data = param.to(device_from_device_id)
                if param.grad is not None:
                    param.grad.data = param.grad.to(device_from_device_id)
        for buffer in buffers:
            buffer.data = buffer.to(device_from_device_id)
    elif current_device == cpu_device:  # type: ignore[possibly-undefined]
        _warn_cpu_init()


def _warn_cpu_init():
    warnings.warn(
        "The passed-in `module` is on CPU and will thus have FSDP's sharding "
        "initialization run on CPU, which may be slower than on GPU. We "
        "recommend passing in the `device_id` argument for FSDP to move "
        "`module` to GPU for the sharding initialization. `module` must also "
        "be on GPU device to work with the `sync_module_states=True` flag "
        "since that requires GPU communication."
    )


def _get_compute_device(
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
    device_from_device_id: Optional[torch.device],
    rank: int,
    device_handle: _FSDPDeviceHandle,
) -> torch.device:
    """
    Determine and return this FSDP instance's compute device.

    If the module is already on a non-CPU device, then the compute device is that non-CPU
    device. If the module is on CPU, then the compute device is the current
    device.

    Since this method should be called after materializing the module, any
    non-CPU device should not be meta device. For now, the compute device is
    always a CUDA or CUDA-like device with its explicit index.

    Precondition: ``_check_single_device_module()`` and
    ``_move_module_to_device()``.
    """
    param = next(_get_orig_params(module, ignored_params), None)
    if param is not None and param.device.type != "cpu":
        compute_device = param.device  # Determined by model param placement
    else:
        compute_device = torch.device(device_handle.current_device())
    if device_from_device_id is not None and compute_device != device_from_device_id:
        raise ValueError(
            f"Inconsistent compute device and `device_id` on rank {rank}: "
            f"{compute_device} vs {device_from_device_id}"
        )
    return compute_device


# TODO: See how to deprecate!
def _sync_module_params_and_buffers(
    module: nn.Module,
    params: List[nn.Parameter],
    process_group: dist.ProcessGroup,
) -> None:
    """
    Synchronize module states (i.e. parameters ``params`` and all not-yet-synced buffers) by broadcasting from rank 0 to all ranks.

    Precondition: ``sync_module_states == True`` and ``self.process_group`` has
    been set.
    """
    module_states: List[torch.Tensor] = []
    for buffer in module.buffers():
        # Avoid re-synchronizing buffers in case of nested wrapping
        if not getattr(buffer, FSDP_SYNCED, False):
            setattr(buffer, FSDP_SYNCED, True)
            detached_buffer = buffer.detach()
            if is_traceable_wrapper_subclass(detached_buffer):
                # NOTE: Here we assume no nested subclasses, at most one level of subclass
                # in both model's buffers and params
                attrs, _ = detached_buffer.__tensor_flatten__()  # type: ignore[attr-defined]
                inner_buffers = [getattr(detached_buffer, attr) for attr in attrs]
                module_states.extend(inner_buffers)
            else:
                module_states.append(detached_buffer)

    for param in params:
        detached_param = param.detach()
        if is_traceable_wrapper_subclass(detached_param):
            attrs, _ = detached_param.__tensor_flatten__()  # type: ignore[attr-defined]
            inner_params = [getattr(detached_param, attr) for attr in attrs]
            module_states.extend(inner_params)
        else:
            module_states.append(detached_param)

    _check_module_states_for_sync_module_states(module_states)
    _sync_params_and_buffers(
        process_group,
        module_states,
        PARAM_BROADCAST_BUCKET_SIZE,
        src=0,
    )


def _check_module_states_for_sync_module_states(
    module_states: List[torch.Tensor],
) -> None:
    if module_states and any(
        tensor.device == torch.device("cpu") for tensor in module_states
    ):
        raise ValueError(
            "The module has CPU parameters or buffers when `sync_module_states=True`, "
            "which requires them to be on GPU. Please specify the `device_id` argument "
            "or move the module to GPU before passing it to FSDP."
        )


def _get_orig_params(
    module: nn.Module,
    ignored_params: Set[nn.Parameter],
) -> Iterator[nn.Parameter]:
    """
    Return an iterator over the original parameters in ``module``.

    The iterator does not return
    the parameters in ``ignored_params``, any ``FlatParameter`` s (which may be
    present due to nested FSDP wrapping), or any original parameters already
    flattened (only relevant when ``use_orig_params=True``).
    """
    param_gen = module.parameters()
    try:
        while True:
            param = next(param_gen)
            if param not in ignored_params and not _is_fsdp_flattened(param):
                yield param
    except StopIteration:
        pass


def _check_orig_params_flattened(
    fsdp_module,
    ignored_params: Set[nn.Parameter],
) -> None:
    """
    Check that original parameters in ``fsdp_module`` have been flattened.

    The flattened parameters are made
    invisible to ``named_parameters()`` for the module hierarchy rooted at
    ``fsdp_module``. This should be called as a sanity check after flattening
    the wrapped module's parameters.
    """
    for param_name, param in _named_parameters_with_duplicates(fsdp_module):
        if param not in ignored_params and not _is_fsdp_flattened(param):
            raise RuntimeError(
                f"Found an unflattened parameter: {param_name}; "
                f"{param.size()} {param.__class__}"
            )


def _get_default_comm_hook(sharding_strategy: ShardingStrategy):
    return (
        default_hooks.allreduce_hook
        if sharding_strategy == ShardingStrategy.NO_SHARD
        else default_hooks.reduce_scatter_hook
    )


def _get_default_comm_hook_state(
    process_group: dist.ProcessGroup,
) -> default_hooks.DefaultState:
    return default_hooks.DefaultState(process_group=process_group)