[Examples] Add an official audio classification example
This PR adds an example for fine-tuning Wav2Vec2-like models on any labeled audio dataset.

The single-GPU script runs on one V100 and trains on [SUPERB KS](https://huggingface.co/datasets/superb#ks)
The multi-GPU one runs on 4 V100s and trains on [CommonLanguage](https://huggingface.co/datasets/anton-l/common_language) (`datasets` PR: https://github.com/huggingface/datasets/pull/2989)
Added a multi-gpu example and trained the demo models:
* https://huggingface.co/anton-l/wav2vec2-base-keyword-spotting
* https://huggingface.co/anton-l/wav2vec2-base-langid

Ready for a full review now :)<|||||>Very nice addition!