Support save/load ckpt for XLA FSDP
Author: yitongh
Pull request: yes

# What does this PR do?

Fixes #32310

Currently, FSDP on XLA only saves the sharded weights for rank 0. This PR enables the saving of complete weights, and also supports the functionality of resuming from a checkpoint. Since FSDP on XLA currently does not support loading a full optimizer into a sharded optimizer, this PR only saves the sharded optimizer and some related sharded metadata to facilitate subsequent loading. With this PR and a corresponding [PR](https://github.com/huggingface/accelerate/pull/2966) for accelerate, FSDP XLA can now support the functionality of resuming from a checkpoint.


## Before submitting
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## Who can review?

@muellerzr and @SunMarc

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_32311). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

hi, @muellerzr , can you help take another look at this PR? I've made some necessary modifications.

Passing off to @ArthurZucker for final :)