Add documentation for multi-label classification
Author: gsnidero
Pull request: yes

# What does this PR do?

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Works toward fixing #9772

The  `problem_type="multi_label_classification"` in [PretrainedConfig](https://huggingface.co/transformers/main_classes/configuration.html#transformers.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



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## 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.


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Comments:
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?

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.




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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Hi @sgugger,

Modified as suggested - hope this helps.

Best,
Giacomo