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Summary:
Update ffmpeg discovery logic

Previously the build process used pkg-config to locate
an installation of ffmpeg, which does not work well Windows/CentOS.

This commit update the discovery process to use the custom
FindFFMPEG.cmake adopted from Kitware/VTK repository with addition of
conda environment.

 The custom discovery logic can support Windows and CentOS.

Pull Request resolved: https://github.com/pytorch/audio/pull/2124

Reviewed By: carolineechen

Differential Revision: D33429564

Pulled By: mthrok

fbshipit-source-id: 6cb50c1d8c58f51e0f3f3af5c5b541aa3a699bba