[ProphetNet] Fix gradient checkpointing bug
Author: yhl48
Pull request: yes

# What does this PR do?

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Fixes #21737 


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. -- #21737 
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- text models: @ArthurZucker and @younesbelkada
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Comments:
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?

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

cc @gante 

Hey @yhl48 -- same comment as in [here](https://github.com/huggingface/transformers/pull/21772#issuecomment-1443568457) (and the other PR has to be merged first) :)

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