Making `transformers` work on `0.12`.
# What does this PR do?

tokenizers `0.12` changed the way `decoder.decode(` works.

Instead of returning directly a string, it returns a list of strings (the "decoded" parts), which enables the decoders to be chained (and hence customized more easily).

The fix works by simply joining those parts for versions >= 0.12.

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Fixes #https://github.com/huggingface/transformers/issues/16520


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Models:

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_The documentation is not available anymore as the PR was closed or merged._<|||||>Thank you very much for the proposed fix. 

Before reviewing this PR, I would like to see if it is possible to keep the new feature in `tokenizers==0.12.0` that allows to chain decoders but to add the conversion to string format at the end. <|||||>Summarizing an oral discussion around our options:

TL;DR


Ultimately the balance between `3/` and `1/` should be the biggest factors in the decision between :
**reverting the change** and preventing us from using the capability which is sometimes needed .
 **Using this PR's change** and making a forward incompatibilty: `transformers<=4.17` incompatible with `tokenizers>=0.12`

1/ The change in `tokenizers` is a good one. On several occasions (last in date CLIP: https://github.com/huggingface/transformers/blob/main/src/transformers/models/clip/tokenization_clip_fast.py#L111) the inability to chain decoders had to be worked around, which was not a great dev UX in `transformers`. It was also a limitation for Bigscience tokenizer (not the latest one, but the one before) and the reason for the change. 

2/  The `"".join(...decoders.decode(tokens))` is a bit clunky and not super self evident.
This is how the coders have to operate now, so using them in isolation should work that way in order to make the composition understandable. There are already discussions around `convert_tokens_to_string` to make it private later, since it causes some issues. Within `tokenizers` itself, there's no way to access `tokens` directly anyway, so users shouldn't have to `.join` in the first place.

A potential less clunky way would be to add `Tokenizer.decode_tokens(tokens)` within `tokenizers` to prevent the join in `transformers` which is indeed clunky. The only issue is that `convert_tokens_to_string` is already causing issues (mostly around the lines like why is `decode` not showing what I think it should) with discussions already going on about making it private at least. Enabling such a function in `tokenizers` might open the same discussions over there. Definitely not a showstopper, but something to think about.

The promoted way `Tokenizers.decode(ids) -> str` remains unchanged for `tokenizers` and so far raised less questions.

3/ Forward compatibility. 

The main caveat to this proposed change, is that earlier versions of `transformers` will contain the bug with the new versions of `tokenizers`. Reverting is the only reasonable solution to fix that (but it's also loosing the composition options we need for the decoders)


4/ Use of `convert_tokens_to_string` in `TokenClassification`.

It's a legacy thing, and is not changed to not break BC, but causes issues on its own: https://github.com/huggingface/transformers/issues/15785#issuecomment-1049040678

Using `offsets` instead of `decode` would help in that situation (we can do it in a non breaking matter by adding a new key, and keeping the old one).

5/ Tokenizer tests
The `transformers`  tokenization tests totally skip that function, which lead to not seeing that function being broken. We're going to update that change to include at least a type test for that function 

<|||||>Would it be possible to consider a deprecation cycle for the tokenizer change, for example with an opt-in flag for the new behavior? Doing this would both keep all previous versions working with 0.12.0, while providing support for the new behavior.

This would allow us to prepare for the breaking change and have at least a few versions that support this before dropping support of the current behavior.<|||||>Ok `3/` forward compatibility is too important to break.

Will revert in `0.12.1` and find cleaner solution for `0.12.2` without breaking things in that manner.