Add Fine-Tuning for Wav2Vec2
# What does this PR do?

This PR adds the possibility to finetune Wav2Vec2 on a downstream task. I ran a couple of experiments and I think the training is pretty stable now, see this training run *e.g.*:

https://wandb.ai/patrickvonplaten/huggingface/reports/Project-Dashboard--Vmlldzo0OTI0OTc?accessToken=8azw8iyxnbiqd4ytxcgm4hbnfh3x1b2c9l2eyfqfzdqw7l0icreljc9qpx0rkl6f

Once this is merged, I will make a nice forum post and link 1,2 notebooks.

## Who can review?

Would be great if @sgugger @LysandreJik and @patil-suraj could review.
This is really nice, and the piece that will make the wav2vec 2.0 stuff awesome and more readily available! let me know if I can assist in testing/whatnot :)