fix resume fsdp
Author: qywu
Pull request: yes

# What does this PR do?

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Fixes [# 23034](https://github.com/huggingface/transformers/issues/23034)

When training a model with FSDP, the checkpoint is not saved and loaded correctly. Only rank 0's optimizer state dict is saved. This PR fixes this issue.

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@pacman100 

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Please run `make style` and `make quality` to fix the quality issues

I have fixed the issues. The optimizer saving had no problems. For using [scatter_full_optim_state_dict](https://pytorch.org/docs/stable/fsdp.html#torch.distributed.fsdp.FullyShardedDataParallel.scatter_full_optim_state_dict), indeed loading on rank 0 is enough, which can save CPU memory usage.

cc @sgugger for a second look

thanks for the fix!