Add the `AudioClassificationPipeline`
# What does this PR do?

This adds the audio classification pipeline needed for `Wav2Vec2ForSequenceClassification` and others (see #13153).

The implementation is mostly based on `ImageClassificationPipeline` with `ffmpeg` audio file loading borrowed from `AutomaticSpeechRecognitionPipeline` by @Narsil 

Once merged, model cards like https://hf.co/superb/hubert-base-superb-ks should be able to have an `audio-classification` inference widget.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?

@patrickvonplaten @Narsil 