GitLab

Summary:
Fix build_doc job

https://app.circleci.com/pipelines/github/pytorch/audio/15217/workflows/ce50b317-a59e-4741-b8d2-59129420deb8

- build.ffmpeg.html might not exist when IPython notebook is processed. Changing to main doc URL.
- Fix bash cell syntax in HW tutorial
- Fix C++ doc
- Fix duplicated target name in streamwriter tutorial

Pull Request resolved: https://github.com/pytorch/audio/pull/3125

Reviewed By: xiaohui-zhang

Differential Revision: D43724078

Pulled By: mthrok

fbshipit-source-id: ea7d46ec5e377cf2fbd7c3798df57da73750ac5c