Add documentation for multi-label classification

[gsnidero]

What does this PR do?

Works toward fixing #9772

The problem_type="multi_label_classification" in PretrainedConfig exist for many models; however, I added documentation for DistilBert only for now. If what I've done so far looks good, I'll add the same to the remaining models; otherwise I need some guidance.

Also, I have added a notebook with a full example in huggingface/notebooks#102

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
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• Did you make sure to update the documentation with your changes? Here are the
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  here are tips on formatting docstrings.

Who can review?

Anyone in the community is free to review the PR once the tests have passed.

@LysandreJik and @sgugger may be interested in this review.

[sgugger]

I don't think this is the right way to fix it: the docstring you have written is great, but it should be added in the base sequence classification docstring, as all models with a sequence classification head should accept problem_type. I will fix the models that do not deal with it and then you can amend your PR, does that sound right?

[gsnidero]

Thanks @sgugger !

Sure - by the "base sequence classification docstring" you mean this?
https://github.com/huggingface/transformers/blob/master/src/transformers/file_utils.py#L828
https://github.com/huggingface/transformers/blob/master/src/transformers/file_utils.py#L1273

Happy to help you fixing the models that do not currently support problem_type if you like. I took the list of models that should support it from https://github.com/huggingface/transformers/blob/master/src/transformers/configuration_utils.py#L181.

[sgugger]

Yes, I meant that docstring.
The PR to enable problem_type on all sequence classification models is #14180 , which will hopefully be merged soon :-)

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[gsnidero]

Hi @sgugger,

Modified as suggested - hope this helps.

Best,
Giacomo

[sgugger]

Thanks a lot for adapting it, it's much better this way!