[Needs Discussion] [WIP] [Docs] Clean tokenizer doc api and add fast tokenizers
Author: patrickvonplaten
Pull request: yes

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Currently, most fast tokenizers do not have their docstrings in the doc. In this PR I want to add all the fast tokenizer docs. Also, I think that the main slow tokenizer `__call__` method should also be added to the docs. @sgugger - before proceeding with all other tokenizer docs, I'd like to hear your opinion on which functions should be included in the docs and which should not.

I'd like to add the `__call__` function to all slow tokenizer docs as well as for all fast tokenizer docs. @sgugger what other functions do you think I should add to the fast tokenizer doc?  


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Comments:
Yes, the person that added the fast sentencepiece tokenizers did not document them in the same PR...
As for the `__call__` method (and the `encode` one), the idea was to refer to the doc of the superclass, but it makes sense to have them directly accessible for each tokenizer. If we add it, we should also add `encode` I think.

> Yes, the person that added the fast sentencepiece tokenizers did not document them in the same PR...
> As for the `__call__` method (and the `encode` one), the idea was to refer to the doc of the superclass, but it makes sense to have them directly accessible for each tokenizer. If we add it, we should also add `encode` I think.

Ok great, I'll add `__call__` and `encode` to all tokenizers. Do you think I should add `encode_plus` and `batch_encode_plus` then as well? Or would that clutter the docs too much in your opinion?

I think `encode_plus` and `batch_encode_plus` are more or less deprecated and should not be in the docs.