Pretraining for TFWav2Vec2
# 🚀 Feature request

TFWav2Vec2 needs the pretraining implementation like the PyTorch [version](https://huggingface.co/transformers/master/model_doc/wav2vec2.html#wav2vec2forpretraining)

## Motivation

Users of the TensorFlow model will most likely want to be able to do pretraining just like with Pytorch. 

## Your contribution

I recently added the [tensorflow model](https://github.com/huggingface/transformers/pull/11617). So I would like to do this one as well. 
