Adding `AutomaticSpeechRecognitionPipeline`.
Author: Narsil
Pull request: yes

# What does this PR do?

- Because we added everything to enable this pipeline, we probably
should add it to `transformers`.
- This PR tries to limit the scope and focuses only on the pipeline part
(what should go in, and out).
- The tests are very specific for S2T and Wav2vec2 to make sure both
architectures are supported by the pipeline. We don't use the mixin for
tests right now, because that requires more work in the `pipeline`
function (will be done in a follow up PR).
- Unsure about the "helper" function `ffmpeg_read`. It makes a lot of
  sense from a user perspective, it does not add any additional
dependencies (as in hard dependency, because users can always use their
own load mechanism). Meanwhile, it feels slightly clunky to have so much
optional preprocessing.
- The pipeline is not done to support streaming audio right now.

# Future work:

- Add `automatic-speech-recognition` as a `task`. And add the
FeatureExtractor.from_pretrained within `pipeline` function.
- Add small models within tests
- Add the Mixin to tests.
- Make the logic between ForCTC vs ForConditionalGeneration better.


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## Who can review?

@LysandreJik
@patrickvonplaten

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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Library:

- benchmarks: @patrickvonplaten
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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
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- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
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Comments:
> Thanks for working on it!
> 
> It is very specific to S2T and Wav2Vec2 but I don't think that's too much of an issue, we can adapt later.
> 
> Could you add this pipeline to:
> 
>     * the documentation

Yes !
> 
>     * the main init

Yes

> 
>     * the `pipeline` factory method

I wanted to defer this into a follow-up PR, because of the AutoModel quirckness and to avoid making super long PRs.
If you feel we can't have it as a separate PR, I'll start including the work directly here.
> 
> 
> We will also probably need a new auto model.