[Flax] Add wav2vec2
# What does this PR do?

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Adds Wav2Vec2 in Flax

- [x] FlaxCTCWav2Vec2
- [x] FlaxForPreTraining
- [x] FlaxWav2Vec2 random mask code
- [x] Clean-up
- [x] Write pretraining script
Anything I can help with ? 
Looking forward to use Flax Wav2vec for the community event :)<|||||>Hey @ThomAub - that's very nice of you! I will need to spend a couple more hours on this and then add pretraining as well. One thing that would be very helpful would be to check how to do the GumbelSoftmax in Flax. *I.e.* how can we translate this: https://github.com/huggingface/transformers/blob/f2c4ce7e339f4a2f8aaacb392496bc1a5743881f/src/transformers/models/wav2vec2/modeling_wav2vec2.py#L751 PyTorch module to Flax? This might be a bit difficult and require some googling to see if others have already implement gumbel softmax in jax/Flax or not. If you could take a look at this, it would be very useful! <|||||>I guess I wasn't fast enough ! Great work <|||||>@patil-suraj @sgugger - actually this is still WIP. Sorry for tagging you too early