Source code for lightning.pytorch.strategies.fsdp

# Copyright The Lightning AI team.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
import logging
import shutil
from contextlib import contextmanager, nullcontext
from datetime import timedelta
from pathlib import Path
from typing import TYPE_CHECKING, Any, Callable, Dict, Generator, List, Literal, Mapping, Optional, Set, Type, Union

import torch
from lightning_utilities.core.rank_zero import rank_zero_only as utils_rank_zero_only
from torch import Tensor
from torch.nn import Module
from torch.optim import Optimizer

import lightning.pytorch as pl
from lightning.fabric.plugins import CheckpointIO, ClusterEnvironment
from lightning.fabric.plugins.collectives.torch_collective import default_pg_timeout
from lightning.fabric.strategies import _StrategyRegistry
from lightning.fabric.strategies.fsdp import (
    _METADATA_FILENAME,
    _activation_checkpointing_kwargs,
    _auto_wrap_policy_kwargs,
    _get_full_state_dict_context,
    _get_sharded_state_dict_context,
    _has_meta_device_parameters,
    _init_cpu_offload,
    _init_sharding_strategy,
    _is_full_checkpoint,
    _is_sharded_checkpoint,
    _load_raw_module_state,
    _move_torchmetrics_to_device,
    _optimizer_has_flat_params,
    _setup_activation_checkpointing,
)
from lightning.fabric.utilities.distributed import (
    _distributed_is_initialized,
    _get_default_process_group_backend_for_device,
    _init_dist_connection,
    _sync_ddp_if_available,
)
from lightning.fabric.utilities.distributed import group as _group
from lightning.fabric.utilities.imports import (
    _TORCH_GREATER_EQUAL_1_13,
    _TORCH_GREATER_EQUAL_2_0,
    _TORCH_GREATER_EQUAL_2_1,
)
from lightning.fabric.utilities.init import _EmptyInit
from lightning.fabric.utilities.load import _lazy_load, _materialize_tensors
from lightning.fabric.utilities.optimizer import _optimizers_to_device
from lightning.fabric.utilities.seed import reset_seed
from lightning.fabric.utilities.types import _PATH, ReduceOp
from lightning.pytorch.core.optimizer import LightningOptimizer
from lightning.pytorch.plugins.precision import Precision
from lightning.pytorch.plugins.precision.fsdp import FSDPPrecision
from lightning.pytorch.strategies.launchers.subprocess_script import _SubprocessScriptLauncher
from lightning.pytorch.strategies.parallel import ParallelStrategy
from lightning.pytorch.strategies.strategy import TBroadcast
from lightning.pytorch.trainer.states import TrainerFn
from lightning.pytorch.utilities.model_helpers import is_overridden
from lightning.pytorch.utilities.rank_zero import rank_zero_info, rank_zero_only, rank_zero_warn

if TYPE_CHECKING:
    from torch.distributed.fsdp.fully_sharded_data_parallel import CPUOffload, MixedPrecision, ShardingStrategy

    if _TORCH_GREATER_EQUAL_2_0:
        from torch.distributed.fsdp.wrap import ModuleWrapPolicy

        _POLICY = Union[Set[Type[Module]], Callable[[Module, bool, int], bool], ModuleWrapPolicy]
    else:
        _POLICY = Union[Set[Type[Module]], Callable[[Module, bool, int], bool]]  # type: ignore[misc]

    _SHARDING_STRATEGY = Union[ShardingStrategy, Literal["FULL_SHARD", "SHARD_GRAD_OP", "NO_SHARD", "HYBRID_SHARD"]]


log = logging.getLogger(__name__)


[docs]class FSDPStrategy(ParallelStrategy):
    r"""Strategy for Fully Sharded Data Parallel provided by torch.distributed.

    .. warning::  This is an :ref:`experimental <versioning:Experimental API>` feature.

    Fully Sharded Training shards the entire model across all available GPUs, allowing you to scale model
    size, whilst using efficient communication to reduce overhead. In practice, this means we can remain
    at parity with PyTorch DDP, whilst scaling our model sizes dramatically. The technique is similar
    to ZeRO-Stage 3.

    For more information check out
    `this blogpost <https://pytorch.org/blog/introducing-pytorch-fully-sharded-data-parallel-api>`__.

    Defaults have been set and options have been exposed, but may require configuration
    based on your level of memory/speed efficiency. We suggest having a look at
    `this tutorial <https://pytorch.org/tutorials/intermediate/FSDP_tutorial.html>`__ for more information.

    Arguments:
        cpu_offload: See ``cpu_offload`` parameter in :class:`torch.distributed.fsdp.FullyShardedDataParallel`.
        mixed_precision: See ``mixed_precision`` parameter in :class:`torch.distributed.fsdp.FullyShardedDataParallel`.
        auto_wrap_policy: Same as ``auto_wrap_policy`` parameter in
            :class:`torch.distributed.fsdp.FullyShardedDataParallel`. For convenience, this also accepts a set of the
            layer classes to wrap.
        activation_checkpointing: Deprecated. Use ``activation_checkpointing_policy``.
        activation_checkpointing_policy: Same as ``auto_wrap_policy`` parameter in
            :class:`torch.distributed.fsdp.FullyShardedDataParallel` but used when selecting the modules for which you
            want to enable activation checkpointing. Enabling this can free up a significant amount of memory at the
            cost of speed since activations in these layers need to be recomputed during backpropagation. For
            convenience, this also accepts a set of the layer classes to wrap.
        sharding_strategy: Select whether to shard model parameters, gradients, optimizer states, or a combination of
            them. Available values are:

            - ``"FULL_SHARD"``: Shards model parameters, gradients, and optimizer states (default).
            - ``"SHARD_GRAD_OP"``: Shards gradients and optimizer states only. Model parameters get replicated.
            - ``"NO_SHARD"``: No sharding (identical to regular DDP).
            - ``"HYBRID_SHARD"``: Shards model parameters, gradients, and optimizer states within a single machine, but
              replicates across machines.

            Also accepts a :class:`torch.distributed.fsdp.ShardingStrategy` enum value.

        state_dict_type: The format in which the state of the model and optimizers gets saved into the checkpoint.

            - ``"full"``: The full weights and optimizer states get assembled on rank 0 and saved to a single file.
            - ``"sharded"``: Each rank saves its shard of weights and optimizer states to a file. The checkpoint is
              a folder with as many files as the world size.

        \**kwargs: See available parameters in :class:`torch.distributed.fsdp.FullyShardedDataParallel`.

    """

    strategy_name = "fsdp"
    _registered_strategies: List[str] = []

    def __init__(
        self,
        accelerator: Optional["pl.accelerators.Accelerator"] = None,
        parallel_devices: Optional[List[torch.device]] = None,
        cluster_environment: Optional[ClusterEnvironment] = None,
        checkpoint_io: Optional[CheckpointIO] = None,
        precision_plugin: Optional[Precision] = None,
        process_group_backend: Optional[str] = None,
        timeout: Optional[timedelta] = default_pg_timeout,
        cpu_offload: Union[bool, "CPUOffload", None] = None,
        mixed_precision: Optional["MixedPrecision"] = None,
        auto_wrap_policy: Optional["_POLICY"] = None,
        activation_checkpointing: Optional[Union[Type[Module], List[Type[Module]]]] = None,
        activation_checkpointing_policy: Optional["_POLICY"] = None,
        sharding_strategy: "_SHARDING_STRATEGY" = "FULL_SHARD",
        state_dict_type: Literal["full", "sharded"] = "full",
        **kwargs: Any,
    ) -> None:
        super().__init__(
            accelerator=accelerator,
            parallel_devices=parallel_devices,
            cluster_environment=cluster_environment,
            checkpoint_io=checkpoint_io,
            precision_plugin=precision_plugin,
        )
        self.num_nodes = 1
        self._process_group_backend = process_group_backend
        self._timeout: Optional[timedelta] = timeout
        self.cpu_offload = _init_cpu_offload(cpu_offload)
        self.mixed_precision = mixed_precision
        self.kwargs = _auto_wrap_policy_kwargs(auto_wrap_policy, kwargs)
        self.sharding_strategy = _init_sharding_strategy(sharding_strategy, self.kwargs)

        if _TORCH_GREATER_EQUAL_2_0:
            # Avoids the need for user to reference params in `configure_optimizers` via
            # `self.trainer.model.parameters()` and enables support for multiple parameter groups.
            self.kwargs.setdefault("use_orig_params", True)

        self._activation_checkpointing_kwargs = _activation_checkpointing_kwargs(
            activation_checkpointing, activation_checkpointing_policy
        )

        if state_dict_type == "sharded" and not _TORCH_GREATER_EQUAL_2_0:
            raise NotImplementedError(
                "Saving checkpoints with `FSDPStrategy(state_dict_type='sharded')` is not supported in PyTorch < 2.0."
                " Please upgrade `torch`."
            )
        self._state_dict_type = state_dict_type

    @property
    def root_device(self) -> torch.device:
        assert self.parallel_devices is not None
        return self.parallel_devices[self.local_rank]

    @property
    def num_processes(self) -> int:
        return len(self.parallel_devices) if self.parallel_devices is not None else 0

    @property
    def process_group_backend(self) -> Optional[str]:
        return self._process_group_backend

    @property
    def mixed_precision_config(self) -> Optional["MixedPrecision"]:
        if self.mixed_precision:
            return self.mixed_precision
        plugin = self.precision_plugin
        if isinstance(plugin, FSDPPrecision):
            return plugin.mixed_precision_config
        return None

    @property  # type: ignore[override]
    def precision_plugin(self) -> FSDPPrecision:
        plugin = self._precision_plugin
        if plugin is not None:
            assert isinstance(plugin, FSDPPrecision)
            return plugin
        return FSDPPrecision("32-true")

    @precision_plugin.setter
    def precision_plugin(self, precision_plugin: Optional[FSDPPrecision]) -> None:
        if precision_plugin is not None and not isinstance(precision_plugin, FSDPPrecision):
            raise TypeError(
                f"The FSDP strategy can only work with the `FSDPPrecision` plugin, found {precision_plugin}"
            )
        self._precision_plugin = precision_plugin

    @property
    def distributed_sampler_kwargs(self) -> Dict:
        return {"num_replicas": (self.num_nodes * self.num_processes), "rank": self.global_rank}

    @property
    def restore_checkpoint_after_setup(self) -> bool:
        return True

    @property
    def lightning_restore_optimizer(self) -> bool:
        return False

[docs]    def setup_environment(self) -> None:
        log.debug(f"{self.__class__.__name__}: setting up distributed...")
        reset_seed()

        # determine which process we are and world size
        self.set_world_ranks()

        self._process_group_backend = self._get_process_group_backend()
        assert self.cluster_environment is not None
        _init_dist_connection(self.cluster_environment, self._process_group_backend, timeout=self._timeout)
        super().setup_environment()

    def _get_process_group_backend(self) -> str:
        return self._process_group_backend or _get_default_process_group_backend_for_device(self.root_device)

    def set_world_ranks(self) -> None:
        if self.cluster_environment is not None:
            self.cluster_environment.set_global_rank(self.node_rank * self.num_processes + self.local_rank)
            self.cluster_environment.set_world_size(self.num_nodes * self.num_processes)
        # `LightningEnvironment.set_global_rank` will do this too, but we cannot rely on that implementation detail
        # additionally, for some implementations, the setter is a no-op, so it's safer to access the getter
        rank_zero_only.rank = utils_rank_zero_only.rank = self.global_rank

    def _configure_launcher(self) -> None:
        assert self.cluster_environment is not None
        if not self.cluster_environment.creates_processes_externally:
            self._launcher = _SubprocessScriptLauncher(self.cluster_environment, self.num_processes, self.num_nodes)

    def _setup_model(self, model: Module) -> Module:
        """Wraps the model into a :class:`~torch.distributed.fsdp.fully_sharded_data_parallel.FullyShardedDataParallel`
        module."""
        from torch.distributed.fsdp import FullyShardedDataParallel

        if any(isinstance(mod, FullyShardedDataParallel) for mod in model.modules()):
            if _has_meta_device_parameters(model):
                rank_zero_warn(
                    "The model is already wrapped in `FSDP` but there are still parameters on the meta device."
                )
            if "auto_wrap_policy" in self.kwargs:
                # The user has wrapped their submodules manually, don't apply the auto wrap policy.
                rank_zero_warn(
                    "A FSDP `auto_wrap_policy` is set, but the model is already wrapped. The policy will be ignored."
                )
                del self.kwargs["auto_wrap_policy"]
        else:
            log.debug(f"setting up FSDP model with device id: {self.root_device.index}, kwargs: {self.kwargs}")
            model = FullyShardedDataParallel(
                module=model,
                cpu_offload=self.cpu_offload,
                mixed_precision=self.mixed_precision_config,
                sharding_strategy=self.sharding_strategy,
                device_id=self.root_device.index,
                **self.kwargs,
            )

        _move_torchmetrics_to_device(model, self.root_device)

        # activation checkpointing needs to be set up after wrapping the model
        if _TORCH_GREATER_EQUAL_1_13:
            _setup_activation_checkpointing(model, self._activation_checkpointing_kwargs)

        return model

[docs]    def setup(self, trainer: "pl.Trainer") -> None:
        assert self.accelerator is not None
        assert self.model is not None
        self.accelerator.setup(trainer)

        if trainer.state.fn == TrainerFn.FITTING and self._layer_sync:
            self.model = self._layer_sync.apply(self.model)

        # we set the device so that optimizers can be created with distributed comms.
        assert self.lightning_module is not None
        self.lightning_module._device = self.root_device

        if is_overridden("configure_sharded_model", self.lightning_module):
            # legacy: we don't skip setup with the `configure_model` alternative
            rank_zero_info(
                "You have overridden `LightningModule.configure_sharded_model` hook. It will assume that all the layers"
                " are already wrapped for sharding and won't wrap the entire model using `FullyShardedDataParallel`."
            )
        else:
            self.model = self._setup_model(self.model)
        self.barrier()

        self.setup_optimizers(trainer)
        _optimizers_to_device(self.optimizers, self.root_device)

        self.setup_precision_plugin()

[docs]    def setup_optimizers(self, trainer: "pl.Trainer") -> None:
        # If we're setting up for evaluation after fitting, we need to discard the optimizers
        # since we're rewrapping the model, otherwise optimizer param references are no longer valid
        # and subsequent checkpoint saving can fail
        self._reset_optimizers_and_schedulers()

        if self.kwargs.get("use_orig_params"):
            return super().setup_optimizers(trainer)

        invalid_params_error = False
        try:
            # In PyTorch < 2.0, or if `use_orig_params=False` the user needs to do access
            # `self.trainer.model.parameters()` in configure_optimizers()
            super().setup_optimizers(trainer)
        except ValueError as ex:
            if "optimizer got an empty parameter list" not in str(ex):
                raise
            invalid_params_error = True

        if invalid_params_error or any(not _optimizer_has_flat_params(optimizer) for optimizer in self.optimizers):
            # We avoid this limitation in PyTorch >= 2.0 by setting `use_orig_params=True`
            raise ValueError(
                "The optimizer does not seem to reference any FSDP parameters. HINT: Make sure to create the"
                " optimizer after setting up the model by referencing `self.trainer.model.parameters()` in the"
                " `configure_optimizers()` hook."
            )
        return None

[docs]    def model_to_device(self) -> None:
        pass

[docs]    @contextmanager
    def tensor_init_context(self, empty_init: Optional[bool] = None) -> Generator[None, None, None]:
        empty_init_context: Union[torch.device, _EmptyInit, nullcontext]
        if _TORCH_GREATER_EQUAL_2_1 and empty_init:
            # Materialization happens in `setup`. When modules get wrapped by FSDP, the sequence of operations is:
            # 1) materialize module 2) call `reset_parameters()` 3) shard the module.
            # These operations are applied to each submodule 'bottom up' in the module hierarchy.
            empty_init_context = torch.device("meta")
        elif _TORCH_GREATER_EQUAL_1_13:
            empty_init_context = _EmptyInit(enabled=bool(empty_init))
        else:
            empty_init_context = nullcontext()
        with empty_init_context, self.precision_plugin.tensor_init_context():
            yield

[docs]    @contextmanager
    def model_sharded_context(self) -> Generator[None, None, None]:
        log.debug(f"{self.__class__.__name__}: entered model_sharded_context.")
        from torch.distributed.fsdp.fully_sharded_data_parallel import FullyShardedDataParallel
        from torch.distributed.fsdp.wrap import enable_wrap

        with enable_wrap(
            wrapper_cls=FullyShardedDataParallel,
            cpu_offload=self.cpu_offload,
            mixed_precision=self.mixed_precision_config,
            sharding_strategy=self.sharding_strategy,
            device_id=self.root_device.index,
            **self.kwargs,
        ):
            yield

[docs]    def barrier(self, name: Optional[str] = None) -> None:
        if not _distributed_is_initialized():
            return
        if torch.distributed.get_backend() == "nccl":
            torch.distributed.barrier(device_ids=self._determine_device_ids())
        else:
            torch.distributed.barrier()

[docs]    def broadcast(self, obj: TBroadcast, src: int = 0) -> TBroadcast:
        if not _distributed_is_initialized():
            return obj

        obj = [obj]
        torch.distributed.broadcast_object_list(obj, src, group=_group.WORLD)
        return obj[0]

[docs]    def reduce(
        self,
        tensor: Union[Tensor, Any],
        group: Optional[Any] = None,
        reduce_op: Optional[Union[ReduceOp, str]] = "mean",
    ) -> Tensor:
        """Reduces a tensor from several distributed processes to one aggregated tensor.

        Args:
            tensor: the tensor to sync and reduce
            group: the process group to gather results from. Defaults to all processes (world)
            reduce_op: the reduction operation. Defaults to 'mean'/'avg'.
                Can also be a string 'sum' to calculate the sum during reduction.

        Return:
            reduced value, except when the input was not a tensor the output remains is unchanged

        """
        if isinstance(tensor, Tensor):
            return _sync_ddp_if_available(tensor, group, reduce_op=reduce_op)
        return tensor

    def _determine_device_ids(self) -> List[int]:
        return [self.root_device.index]

[docs]    def teardown(self) -> None:
        log.debug(f"{self.__class__.__name__}: tearing down strategy...")

        pl_module = self.lightning_module
        if (
            pl_module is not None
            # `self.lightning_module._trainer` can be None if teardown gets called on an exception before
            # the trainer gets set on the LightningModule
            and pl_module._trainer is not None
            and pl_module._trainer.state.fn == TrainerFn.FITTING
            and self._layer_sync
        ):
            assert self.model is not None
            self.model = self._layer_sync.revert(self.model)

        assert self.cluster_environment is not None
        assert self.accelerator is not None
        self.cluster_environment.teardown()
        self.precision_plugin.teardown()
        self.accelerator.teardown()

    @classmethod
    def get_registered_strategies(cls) -> List[str]:
        return cls._registered_strategies

    @classmethod
    def register_strategies(cls, strategy_registry: _StrategyRegistry) -> None:
        if not torch.distributed.is_available():
            return
        strategy_registry.register(
            "fsdp",
            cls,
            description="Fully Sharded Data Parallel (FSDP) training",
        )
        cls._registered_strategies.append("fsdp")

        strategy_registry.register(
            "fsdp_cpu_offload",
            cls,
            description="Fully Sharded Data Parallel (FSDP) training with Full Sharding and CPU Offloading",
            cpu_offload=True,
        )
        cls._registered_strategies.append("fsdp_cpu_offload")

[docs]    def lightning_module_state_dict(self) -> Dict[str, Any]:
        assert self.model is not None
        if self._state_dict_type == "sharded":
            state_dict_ctx = _get_sharded_state_dict_context(self.model)
        elif self._state_dict_type == "full":
            state_dict_ctx = _get_full_state_dict_context(self.model, world_size=self.world_size)
        else:
            raise ValueError(f"Unknown state_dict_type: {self._state_dict_type}")
        with state_dict_ctx:
            return self.model.state_dict()

    def load_model_state_dict(self, checkpoint: Mapping[str, Any]) -> None:
        # Override to do nothing, FSDP already loaded the states in `load_checkpoint()`
        pass

[docs]    def optimizer_state(self, optimizer: Optimizer) -> Dict[str, Tensor]:
        if not _TORCH_GREATER_EQUAL_2_0:
            rank_zero_warn("FSDP in Lightning with PyTorch < 2.0 does not support saving the optimizer state.")
            return {}

        from torch.distributed.fsdp import FullyShardedDataParallel as FSDP
        from torch.distributed.fsdp import OptimStateKeyType

        if isinstance(optimizer, LightningOptimizer):
            optimizer = optimizer._optimizer

        assert self.model is not None
        if self._state_dict_type == "sharded":
            with _get_sharded_state_dict_context(self.model):
                return FSDP.optim_state_dict(self.model, optimizer)

        elif self._state_dict_type == "full":
            with _get_full_state_dict_context(self.model, world_size=self.world_size):
                state_dict = FSDP.optim_state_dict(self.model, optimizer)
                if self.global_rank == 0:
                    # Store the optimizer state dict in standard format
                    state_dict = FSDP.rekey_optim_state_dict(state_dict, OptimStateKeyType.PARAM_ID, self.model)
                return state_dict

        raise ValueError(f"Unknown state_dict_type: {self._state_dict_type}")

    def load_optimizer_state_dict(self, checkpoint: Mapping[str, Any]) -> None:
        # Override to do nothing, the FSDP already loaded the states in `load_checkpoint()`
        pass

[docs]    def save_checkpoint(
        self, checkpoint: Dict[str, Any], filepath: _PATH, storage_options: Optional[Any] = None
    ) -> None:
        if storage_options is not None:
            raise TypeError(
                "`FSDPStrategy.save_checkpoint(..., storage_options=...)` is not supported because"
                " `FSDPStrategy` does not use the `CheckpointIO`."
            )

        path = Path(self.broadcast(filepath))
        if path.is_dir() and self._state_dict_type == "full" and not _is_sharded_checkpoint(path):
            raise IsADirectoryError(f"The checkpoint path exists and is a directory: {path}")

        if self._state_dict_type == "sharded":
            from torch.distributed.checkpoint import FileSystemWriter, save_state_dict

            if path.is_file():
                path.unlink()
            path.mkdir(parents=True, exist_ok=True)

            converted_state = {"model": checkpoint.pop("state_dict")}
            converted_state.update(
                {f"optimizer_{idx}": optim_state for idx, optim_state in enumerate(checkpoint.pop("optimizer_states"))}
            )

            # FSDP's FileSystemWriter streams the tensors to disk to minimize memory peaks
            writer = FileSystemWriter(path=path, single_file_per_rank=True)
            save_state_dict(converted_state, writer)

            if self.global_rank == 0:
                torch.save(checkpoint, path / _METADATA_FILENAME)
        elif self._state_dict_type == "full":
            if _is_sharded_checkpoint(path):
                shutil.rmtree(path)
            return super().save_checkpoint(checkpoint=checkpoint, filepath=path)
        else:
            raise ValueError(f"Unknown state_dict_type: {self._state_dict_type}")

    def load_checkpoint(self, checkpoint_path: _PATH) -> Dict[str, Any]:
        # broadcast the path from rank 0 to ensure all the states are loaded from a common path
        path = Path(self.broadcast(checkpoint_path))

        from torch.distributed.fsdp import FullyShardedDataParallel as FSDP

        assert self.model is not None
        assert self.lightning_module is not None

        if _is_sharded_checkpoint(path):
            from torch.distributed.checkpoint import FileSystemReader, load_state_dict
            from torch.distributed.checkpoint.optimizer import load_sharded_optimizer_state_dict

            state_dict_ctx = _get_sharded_state_dict_context(self.model)
            reader = FileSystemReader(path=path)

            with state_dict_ctx:
                module_state = {"model": self.model.state_dict()}
                load_state_dict(module_state, reader)
                self.model.load_state_dict(module_state["model"])

                if self.lightning_module.trainer.state.fn == TrainerFn.FITTING:
                    # the optimizer states must be loaded separately
                    for idx, optim in enumerate(self.optimizers):
                        optim_key = f"optimizer_{idx}"
                        optim_state = load_sharded_optimizer_state_dict(
                            model_state_dict=module_state["model"],
                            optimizer_key=optim_key,
                            storage_reader=reader,
                        )
                        flattened_osd = FSDP.optim_state_dict_to_load(
                            optim_state_dict=optim_state[optim_key],
                            model=self.model,
                            optim=optim,
                        )
                        optim.load_state_dict(flattened_osd)

            # Load metadata (anything not a module or optimizer)
            metadata = torch.load(path / _METADATA_FILENAME)
            return metadata

        if _is_full_checkpoint(path):
            checkpoint = _lazy_load(path) if _TORCH_GREATER_EQUAL_2_0 else torch.load(path, map_location="cpu")
            _load_raw_module_state(checkpoint.pop("state_dict"), module=self.model, world_size=self.world_size)

            if _TORCH_GREATER_EQUAL_2_0:
                # Materialize lazy tensors if there are any left in the checkpoint
                # The `torch.Optimizer.load_state_dict` method can't load lazy tensors because of deepcopy pickle issues
                checkpoint = _materialize_tensors(checkpoint)

            from torch.distributed.fsdp import FullyShardedDataParallel as FSDP
            from torch.distributed.fsdp import OptimStateKeyType

            optimizer_states = checkpoint.get("optimizer_states")
            if optimizer_states is None or self.lightning_module.trainer.state.fn != TrainerFn.FITTING:
                # If the optimizer states are not present, we don't need to do anything (backward compatibility)
                return checkpoint
            if not _TORCH_GREATER_EQUAL_2_0:
                rank_zero_warn("FSDP in Lightning with PyTorch < 2.0 does not support loading the optimizer state.")
                return checkpoint
            if len(self.optimizers) != len(optimizer_states):
                raise RuntimeError(
                    f"You have configured {len(self.optimizers)} optimizers but the checkpoint contains"
                    f" {len(optimizer_states)} optimizers to load. Please resume training with the same number"
                    " of optimizers or edit the checkpoint manually to remove states."
                )

            # rank0_only should be false because we need to load the optimizer state on all ranks
            with _get_full_state_dict_context(self.model, world_size=self.world_size, rank0_only=False):
                for optimizer, opt_state in zip(self.optimizers, optimizer_states):
                    if isinstance(list(opt_state["state"].keys())[0], int):
                        # Handling the case where the optimizer state is saved from a normal optimizer
                        opt_state = FSDP.rekey_optim_state_dict(opt_state, OptimStateKeyType.PARAM_NAME, self.model)

                    opt_state = FSDP.optim_state_dict_to_load(
                        optim_state_dict=opt_state,
                        model=self.model,
                        optim=optimizer,
                    )
                    optimizer.load_state_dict(opt_state)

            return checkpoint

        raise ValueError(
            f"The path {str(path)!r} does not point to a valid checkpoint. Make sure the path points to either a"
            " directory with FSDP checkpoint shards, or a single file with a full checkpoint."
        )