[WIP] Add TFSpeech2Text
Author: will-rice
Pull request: yes

# What does this PR do?

This PR adds TFSpeech2Text. The issue that requested it was recently closed due to inactivity so I don't think it is being worked on currently. If this is an incorrect assessment, feel free to let me know and I will close this. 

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Fixes # (issue)


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@patil-suraj 

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
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- gpt2: @patrickvonplaten, @LysandreJik
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- tensorflow: @LysandreJik

Library:

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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
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HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
Gently pinging @stancld to see if he's still working on this :) 

@patil-suraj I'm having a little trouble with the generate tests. Should I just do a custom generate function for this model in the modeling file or modify the base generate function to accommodate this model? 

Sorry to only reply now.

What is the issue with `generate`? Ideally `generate` should work, so let's try to avoid adding a custom `generate`. 


This [assert](https://github.com/huggingface/transformers/blob/596bb85f2fabde6c5611cfa2664ddb357e228ec7/src/transformers/generation_tf_utils.py#L624) is the beginning of the issue. This is probably my interpretation of `input_ids`. Since the `input_ids` is changed to `input_features`, all of the generate functions pop the `input_ids` key. The shape of `input_features` is 3 dimensions though which most other models have a 2 dimension input for encoder-decoder models. I'm going to try to wrap this up today though so I'll figure out what the issue is without doing a custom `generate`.