fix GPT2 token's `special_tokens_mask` when used with `add_bos_token=True`
Author: SaulLu
Pull request: yes

# What does this PR do?

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Fix: #19035 

This PR allows to correct the mask of special tokens when using the tokenizer of GPT2 with `add_bos_tokens=True`


## 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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      Pull Request section?
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- [x] 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. Would love to have your input on it @sgugger , @LysandreJik , @patrickvonplaten and @ArthurZucker 

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