[ProphetNet] Fix gradient checkpointing bug

[yhl48]

What does this PR do?

Fixes #21737

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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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• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case. -- [Generate] Fix gradient_checkpointing and use_cache bug for generate-compatible models #21737
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[yhl48]

the code quality seems to have failed due to some other files that were not changed, but could anyone please confirm if that is the case?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[sgugger]

cc @gante

[gante]

Hey @yhl48 -- same comment as in here (and the other PR has to be merged first) :)

[gante]

(#21772 contains the changes here, closing this PR)