Patch MistralCommonTokenizer

[juliendenize]

What does this PR do?

This PR patches the MistralCommonTokenizer:

1. [BUG FIX] spm now is correctly supported as previous usage would result in an error for _piece_to_id and _is_control_token.
2. [BUG FIX] previous implementation of _piece_to_id was incorrect for Tekkenizer.

a) the special tokens were not supported
b) the normal tokens were not shifted by adding the number of special tokens

4. [MAYBE BUG FIX] Changed get_vocab to a function that should better mimic what happens in Transformers: now the mapping is based on the real ids but some ids are missing due to conversion loss of some tokens in Tekken.
5. [FEATURE] add_generation_prompt has been added. This is to match signature of Transformers. In practice this value is ignored except if:

a) continue_final_message and add_generation_prompt are True an error is raised.
b) if add_generation_prompt is True and the last message is assistant then an error is raised as the user should have passed continue_final_message.

6. [FEATURE] Now the tokenizer is set to fast because:

a) it is true(Edit: not so sure about that after discussion)
b) it removes annoying message when initializing the tokenizer.

Edit 2: this has been reverted as it doesn't bring value and is misleading.

7. [OPTIMIZATION] now image tensors are converted without slowness warnings from torch.
8. [DOCS] updated some docs to remove unused args.

Also added minimal tests to ensure SPM also works in the future.

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
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• 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?

@patrickvonplaten @ArthurZucker @itazap

[github-actions]

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[ArthurZucker]

Thanks for updating! 🤗

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