Add Speech Seq2Seq Training script
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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An explanation of this new training script is given on the README.md.
Two successful training runs can be seen here: https://huggingface.co/models?other=asr_seq2esq


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [ ] Did you write any new necessary tests?


## Who can review?

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
Hmmm, not that easy the training it seems...will run some more examples next week with Wav2Vec2 - BERT

Getting good results now for wav2vec2 - bart: https://huggingface.co/patrickvonplaten/wav2vec2-2-bart-base

Results are good: https://huggingface.co/models?other=asr_seq2esq 

Cleaning up the PR and we can merge in 1,2 days

Merging this now. Very much agree on the naming issue @sgugger and thanks for reminding me again. Will open another PR for this later today.