GitLab

Summary:
Adding C++ documentation. (C++ APIs are categorized as prototype, though it's used by Python beta APIs.)

https://output.circle-artifacts.com/output/job/69654229-a99e-4b15-9ce0-7bc6bcf01101/artifacts/0/docs/libtorchaudio.html

<img width="1202" alt="Screenshot 2023-01-31 at 11 48 47 AM" src="https://user-images.githubusercontent.com/855818/215828167-d23032f8-9e40-4413-b5b1-5cbd12d705e9.png">

Pull Request resolved: https://github.com/pytorch/audio/pull/2994

Reviewed By: hwangjeff

Differential Revision: D42876621

Pulled By: mthrok

fbshipit-source-id: d8b8d610b87ec766501baa88b7506368a9905a6a