feat: add async decode batch type annotations in decoding

[michaelfeil]

Something I forgot to add to #1843.
Reminder to myself to also wire up native asynchronous support.

[michaelfeil]

@ArthurZucker this should be an easy one to review. TY.

[ArthurZucker]

yeah sure, I don't think its used much but let' s go

[HuggingFaceDocBuilderDev]

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[michaelfeil]

Seems like a unrelated failure

[ArthurZucker]

Yep!

[Qubitium]

Hi @ArthurZucker @michaelfeil I just ran into this PR and am curious at the structure: why is BaseTokenizer another wrapper which holds the real tokenzer in _tokenizer? Since the naming and the code is confusing me a little. It's been a long time since I checked the internals of hf tokenizers so I am really clueless and want to start.

[ArthurZucker]

its not an internal its more like an example of how you can use. I don't think many people use it tho