Draft backtrack

[ArthurZucker]

The goal of the PR is to allow anyone to use a BPE serialized model into a BacktrackingBpe class. This way you only have to use the correct class to leverage the new method.

Serialization should be of BacktrackingBpe type if we want to save time on initializing the double aho corasicks. TBH I am not sure it's worth it, so we'll see.

Full credits to https://github.com/github/rust-gems

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[ArthurZucker]

TODOs:

• use a trait for BPETrainer to run on BacktrackingBpe
• implement a converter directly, instead of the deserialization I am currently doing
• implement model.use_backtracking_bpe() for BPE that just returns a converted BacktrackingBpe