Fix conflicting key in init kwargs in PreTrainedTokenizerBase
Author: OmarManzoor
Pull request: yes

# What does this PR do?
Checks if there is any key that matches a method name in PreTrainedTokenizerBase and raises an error if so to prevent errors that occur during serialization.
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Fixes #30796

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Comments:
> LGTM, tho I think if someone tries to save add_special_tokens it's something we would want to save to re-use when encoding. Might be the opposite of what I said, but saving it can be convenient WDYT?

Thanks for the review. It makes sense to save it. How do you propose we should save it though since we might not want to use the exact same name as the method?

@ArthurZucker I added the check for a conflicting method in the save_pretrained method. Is this suitable?

We can set a field `_add_special_tokens` which makes sense, but in that case where this field is used has to be changed? Also for handling this generically this would mean adding an _ to all such fields in init args that conflict with a method name and that might be a bit more complex to handle.

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_31233). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

Could you re-commit something the CI does not seem to start 

@ArthurZucker I am not sure about the failing tests.

Unrelated no worries