Feature Extractor: Wav2Vec2 & Speech2Text - Allow truncation + padding=longest
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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For training is speech it's quite important that we are able to make use of the `padding="longest"` *a.k.a* dynamic batching in PyTorch on GPU since speech inputs can vary from 2 seconds to up to 30 seconds. We also want to be able to use truncation with `padding="longest"` in such a case. This PR enables using truncation (*i.e.* cutting sequences to a max length of x seconds) with the `padding="longest"` mode.


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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
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Library:

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- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
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