Adding support for `pipeline("automatic-speech-recognition")`.
Author: Narsil
Pull request: yes

# What does this PR do?

Implements the default load logic to make `AutomaticSpeechRecognitionPipeline` work
like other pipelines with `pipeline(task="automatic-speech-recognition", ...)`.

The main issue with current implementation is `"config"` choice for AutoModel. It would be great to have the
possibility to have something like `AutoModelFor` that would implement
the same logic (Load the config, check Architectures and load the first
one)

Alternatives:

- Implement `AutoModelForCTC`, `AutoModelForConditionalGeneration`, allow the `ALLOWED_TASKS` mapping
  to accept iterables, and try to load models accordingly.
  This might enable better switch handling case here: https://github.com/huggingface/transformers/blob/master/src/transformers/pipelines/automatic_speech_recognition.py#L141 with actual `isinstance` instead of the dummy string check.
  This would change `ALLOWED_TASKS` logic but might be closer to existing code.


Other discussion could include the `Mixin` which wasn't used here. The main reason is because the mixin supposes
that TF is enabled, but the ASR models do not have a TF alternative right now. Still imported the main tests.

Better care of error raised could be added for the `feature_extractor` missing or incorrect.



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Fixes # (issue)


## Before submitting
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Models:

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Ping


@patrickvonplaten could make another review , I think the previous PR that reads `config.architectures` alleviated any issue for this PR.

what do you think ?

@sgugger

Maybe a small sanity check if you don't mind (code has significantly changed since last review) ? Should be for the better.