fix FSDP + torch.compile bug when saving pretrained model 
Author: Joaquinecc
Pull request: yes

# What does this PR do?
The issue at hand relates to the behavior of `accelerator.unwrap_model` when used with FSDP and `torch.compile`. The problem occurs when the model, wrapped by `torch.compile`, is not unwrapped correctly when saving, leading to a failure to save the model properly. Specifically, the `torch.compile` wrapper isn't removed before saving, causing the model's parameters to be incorrectly modified. The solution to fix the issue was to adjust the `unwrap_model` method to not reapply the `torch.compile` wrapper after FSDP has been applied. 
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Fixes [37519](https://github.com/huggingface/transformers/issues/37519)


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Comments:
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> Thanks ! Can you add a test also ?

Hey @SunMarc  I add a test with FSDP + Torch compile, if you can check it. 

> LGTM ! Please fix style issue with `make style`

Done =)

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_37725). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Sorry for the last bad commit, I undo it and fix it. is there anything to add to aprove this PR? @SunMarc @S1ro1 @jiangyukunok @zach-huggingface

Waiting for @S1ro1 review !