Remove the multi step tokenization warning when using HF Data Collators
We create a new function that pads without warning the user to instead calling `tokenizer.forward` because it's faster.

We use it for Transformers' own DataCollator calls.

It doesn't make much sense that a DataCollator would change the state of a tokenizer imho, so every time: 
    - we save the state of the tokenizer with regards to the warning
    - disable the warning
    - pad
    - restore the state of whether we want to warn or not.

See https://github.com/huggingface/transformers/issues/22638
@sgugger<|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_23742). All of your documentation changes will be reflected on that endpoint.<|||||>Doing things this way could lead to weird (though not dangerous) race behaviour in threaded code.

The only way I see to not get that race behavior is to let `pad` take an extra argument to disable the warning, though I understand that API providers are very hesitant to alter APIs in any way
 <|||||>The race behavior would just be that the warning would not be displayed ofc<|||||>The data collator cannot really be in threaded code. Different processes? Definitely for distributed training but then they will each have their own tokenizer. So I think it's a risk we can take.<|||||>I'm sorry @sgugger, you mean the branch name? Or the pull request name? Or the function name? I changed the name of the pull request.

Also, am I supposed to run `make quality; make style` first? & am I supposed to run some tests?<|||||>I synced the branch<|||||>The PR now deletes 26 doc files, so it's not really something we can merge :grimacing: <|||||>OK I fixed the weird document files deletion. Really not sure how that happened. Sorry about that.<|||||>Could you just run `make style` to fix the formatting issues? Thanks!<|||||>done<|||||>There is still an error. Can you make sure to do `pip install transformers["quality"] --upgrade` (to make sure you have a proper version of all the necessary libs)?<|||||>Mmm now we're back to 43 files changed :cry: <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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