Fix tokenizer type annotation
Author: willfrey
Pull request: yes

It should probably accept either `PreTrainedTokenizer` or `PreTrainedTokenizerFast`.

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Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_17592). All of your documentation changes will be reflected on that endpoint.

Could we put it in brackets, so that the import of the class isn't necessary? IDEs will still get it right, and as we want users to be able to leverage `transformers` without `tokenizers` installed on setups that don't support rust, it would be the least breaking thing to do.

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.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.