preserve parameter keys when removing prefix

[mjkvaak-amd]

What does this PR do?

Earlier PR #3560 broke FSDP2 for non-compiled models due to omitted keys from the named_params dict. This change fixes the mistake in the if-replace logic, by conditionally stripping the "_orig_mod." prefix only when it's present as intended.

Fixes #3554

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@S1ro1, @SunMarc

[SunMarc]

Thanks for noticing this !

[HuggingFaceDocBuilderDev]

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[SunMarc]

@bot /style

[github-actions]

Style fixes have been applied. View the workflow run here.

[S1ro1]

Thank you for the catch! LGTM.