

TITLE
Add TFSpeech2Text

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# What does this PR do?

This PR adds a TF port of Speech2Text. A summary of the changes:
- This model borrows a lot of code from TFBart, just like Speech2Text borrowed from Bart;
- Tried to follow the changes in other PRs to enable smooth interoperation with other parts of transformers (e.g. auto classes), might be missing a few things 👼 ;
- This seems to be the first TF model with speech as input, so I had to touch common TF code to enable correct data piping and misc operations (e.g. enable loading Conv1D PT weights into TF);
- Likewise, there were a few tests in `test_modelling_tf_common.py` that didn't quite fit this new kind of model.

TODO: 
- [x] create TF version of the weights, so we can load a model without `from_pt=True`