Allowing adding new token as unk token for gpt2 tokenizer

[lsy641]

What does this PR do?

Fixes # (issue) #https://github.com/huggingface/transformers/issues/22414

Before submitting

• [yes ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [yes] Did you read the contributor guideline, Pull Request section?
• [yes] Was this discussed/approved via a Github issue or the forum? Please add a link to it if that's the case.
• [ ] Did you make sure to update the documentation with your changes? Here are the documentation guidelines, and here are tips on formatting docstrings.
• [ no] Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag members/contributors who may be interested in your PR. @ArthurZucker

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint.

[Narsil]

I think we should add some tests to clarify what behavior is modified an how.

It could be for just those 4 tokenizers, but still I think the effect of this PR is not entirely clear by just reading it.

[ArthurZucker]

Also let's make sure that the tests are all green !

[lsy641]

What do you need me to do to help this merge?

[ArthurZucker]

Hey! As mentioned in my last comments, the CI tests need to be all green 😉 Mostly make fixup should help you

[ArthurZucker]

I remember testing for a few models and having some issues with this update in token addition, I'll have to check once the PR is ready

[github-actions[bot]]

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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