Deprecate Wav2Vec2ForMaskedLM and add Wav2Vec2ForCTC

[patrickvonplaten]

What does this PR do?

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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[LysandreJik]

LGTM! Could we remove Wav2Vec2ForMaskedLM from the documentation?

[patrickvonplaten]

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!