GitLab

Summary:
Preparation to land Python front-end of ffmpeg-related features.

- Set BUILD_FFMPEG=1 in Linux/macOS unit test jobs
- Install ffmpeg and pkg-config from conda-forge
- Add note about Windows build process
- Temporarily avoid `av_err2str`

Pull Request resolved: https://github.com/pytorch/audio/pull/2114

Reviewed By: hwangjeff

Differential Revision: D33371346

Pulled By: mthrok

fbshipit-source-id: b0e16a35959a49a2166109068f3e0cbbb836e888