Add `from_slow` in fast tokenizers build and fixes some bugs
# What does this PR do?

This PR adds an argument to the initialization of the `PreTrainedTokenizerFast` to force the conversion from a slow tokenizer. This will be useful to help users re-build the `tokenizer.json` file for some models where we can't update faulty ones right now without breaking backward compatibility (see #9637).

In passing it fixes a few bugs:
- wrong formatting for the documentation
- the fast sentencepiece tokenziers don't have an `sp_model` attribute so remove the documentation for that
- BarthezTokenizerFast was not registered properly in the autotokenizers, so `AutoTokenizer` was not finding it