What does this PR do?

Fixes the behavior of add_special_tokens to match PretrainedTokenizer goals:

• if mode is finetuning: add both bos and eos tokens
• if mode is test: add only bos tokens so that the model can generate freely.

Also adds the possibility to use ValidationMode as a string to avoid import from mistral-common for the user.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• 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
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  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

cc @patrickvonplaten for viz

@ArthurZucker and @itazap