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Summary:
Since libsox and ffmpeg extensions now depend on external libraries, their initialization processes might cause unrecoverable issue, such as segfault.

This commit adds environment variable to disable them so that importing torchaudio won't attempt to load these libraries.

Pull Request resolved: https://github.com/pytorch/audio/pull/3500

Differential Revision: D47808178

Pulled By: mthrok

fbshipit-source-id: 80c1c6b5f4bc608d4e209473702680db093c95ee