remove FSDP prefix when using save_pretrained with FSDP2
Author: winglian
Pull request: yes

# What does this PR do?

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Fixes # (issue)


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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_40207). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Can you check the failing test? Seems related

I see this bug fix in 4.56.1-Vault-Gemma-preview release, but not in 4.56.2. Not sure why? @S1ro1