[Whisper] Add model for audio classification
# What does this PR do?

Adds `WhisperForAudioClassification`: the Whisper encoder model with a sequence classification head on top for audio classification tasks.

With the changes implemented in #21756, Whisper can be fine-tuned for any audio classification task.

Results of fine-tuning for suggest that this is an extremely promising approach for audio classification. On the FLEURS language identification task, fine-tuning Whisper medium achieves an accuracy of **88%**, beating previous SoTA by 10% and the zero-shot Whisper model by 24% absolute:

<img width="614" alt="Screenshot 2023-02-28 at 09 37 27" src="https://user-images.githubusercontent.com/93869735/223370179-338e1f12-7793-48e5-9715-66d7f9da4af3.png">

See logs at [sanchit-gandhi/whisper-medium-fleurs-lang-id](https://huggingface.co/sanchit-gandhi/whisper-medium-fleurs-lang-id) for details.
_The documentation is not available anymore as the PR was closed or merged._<|||||>(Failing test is unrelated)