Deprecate Wav2Vec2ForMaskedLM and add Wav2Vec2ForCTC
Author: patrickvonplaten
Pull request: yes

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Deprecates `Wav2Vec2ForMaskedLM` -> the name was very badly chosen since it's currently used for CTC classification which is very different from `MaskedLM`. Also `MaskedLM` is not a good name for pretraining where it should rather be something like `ForMaskedSpeechModeling`, so IMO the best idea is to deprecate the whole class. 

Right after this PR is merged and there is a patch, I will update all configs.


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Comments:
> LGTM! Could we remove `Wav2Vec2ForMaskedLM` from the documentation?

Yes! It's better than adding a note saying that the model is deprecated? => yeah let's just remove it!