WIP: Initial FSDP2 support

[S1ro1]

Draft PR, feel free to discuss changes to the user-facing api.

Fixes # (issue)

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[muellerzr]

Great start! Left some initial comments

[muellerzr]

Suggested change

	"reshard_after_forward set to bool. This is not supported in FSDP1, please set to a `str` or an instance of `torch.distributed.fsdp.fully_sharded_data_parallel.ShardingStrategy`"
	f"reshard_after_forward set to {self.reshard_after_forward}. This is not supported with FSDP1, please set to a `str` or an instance of `torch.distributed.fsdp.fully_sharded_data_parallel.ShardingStrategy`"

[muellerzr]

I feel like we can simplify this

Suggested change

	if self.fsdp_version != 2 and isinstance(self.cpu_offload, CPUOffloadPolicy):
	raise ValueError(
	"cpu_offload set to `torch.distributed.fsdp.CPUOffloadPolicy`. This is not supported in FSDP1, please set to a `bool` or an instance of `torch.distributed.fsdp.CPUOffload`"
	)
	if self.fsdp_version == 2 and not isinstance(self.cpu_offload, CPUOffloadPolicy):
	raise ValueError(
	"cpu_offload set to `bool` or `torch.distributed.fsdp.CPUOffload`. This is not supported in FSDP2, please set to an instance of `torch.distributed.fsdp.CPUOffloadPolicy`"
	)
	if isinstance(self.cpu_offload, CPUOffloadPolicy):
	err = "`cpu_offload` set to `torch.distributed.fsdp.CPUOffloadPolicy`."
	if self.fsdp_version != 2:
	raise ValueError(f"{err} This is not supported in FSDP1, please set to a `bool` or an instance of `torch.distributed.fsdp.CPUOffload`")
	else:
	raise ValueError(f"{err} This is not supported in FSDP2, please set to an instance of `torch.distributed.fsdp.CPUOffloadPolicy`")

[muellerzr]

Note that in general, users will ignore warnings (and will get annoyed it bloats the logs). So we should instead use logger.warn if we don't want to explicitly raise an error about this.

[muellerzr]

Instead of doing many warning.warn,s let's accumulate them all and do one big logger.warn at the end

[muellerzr]

Suggested change

	if self.fsdp_version == 2 and self.forward_prefetch is not None:
	raise ValueError(
	"forward_prefetch is not yet implemented in FSDP2, set to None or use `fsdp_version` set to 1"
	)
	if self.fsdp_version == 2 and self.forward_prefetch is not None:
	raise ValueError(
	"forward_prefetch is not yet implemented in FSDP2, set to None or use `fsdp_version=1`"
	)

[muellerzr]

Will this import lead to issues on old pytorch versions?

[kmehant]

@muellerzr @S1ro1

I see potential collaboration on this thread with my PRs

1. Support TP + FSDPv2 / HSDP or just FSDPv2 / HSDP #3395
2. [RFC] Support FSDP2 #3231

With changes on this PR I understand that the design is to convert FSDP1 args to FSDP2 args and put to use. With my initial discussions started at #3231, In the PR #3395 I propose to have FSDP2 as a separate distributed type training protocol alongsides FSDP1. Also, to support more complex combinations of parallelisms I as well propose a design as described in the PR's description.

Looking forward.