Fixing a regression with `return_all_scores` introduced in #17606
Author: Narsil
Pull request: yes

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 Fixing a regression with `return_all_scores` introduced in #17606

    - The legacy test actually tested `return_all_scores=False` (the actual
      default) instead of `return_all_scores=True` (the actual weird case).

    This commit adds the correct legacy test and fixes it.

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Fixes # (issue)


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Comments:
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@sgugger Do you mind just re-reviewing, I pushed the PR a little early yesterday (forgot WIP tag).

The real edge case (present in `4.19.4`) is when there's a single incoming item and `return_all_scores=True`, somehow we are always returning `[CLASS_DICT]` but when using `return_all_scores` we're returning `[[CLASS_DICT, CLASS_DICT]]` .

I don't fully remember why the second list popped from the very old legacy code (before the pipeline rework) but that's the reason for the weird return type in the beginning.

IMO, we should return ALWAYS a list when classifying a single text ( containing only the top element by default `, which is fully backward compatible).

Then, we keep the odd LIST of LIST when using `return_all_scores=True` (BC, + add a warning to move to `top_k`.)

Then we change the return when using `top_k=None` or `top_k=n` to contain a single LIST of the classes (so more aligned with the return type without any parameters).

WDYT about this solution ?

Do you think we should be more conservative and keep LIST of LISTS all the time (even when using the new parameter )?.
(The API itself will maintain that return legacy type, while I go look if we can update the widget itself)

Also, when sending a LIST of str as an input the output will ALWAYS be a list of list of classes in all scenarios.


PS: Failing tests seem to be linked with new `black` version so I am going to ignore them and rebase later.

Mmm I see weird changes in `molideng_utils` and `gpt2` now. I think your solution is sensible.

Wrong rebase on my end.