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Summary:
Originally, the global property TORCHAUDIO_THIRD_PARTIES was introduced
to handle the optional third party dependencies that can change based on
the build config.

After revising the CMake, it turned out this is not really necessary,
as our torchaudio/csrc/CMakeLists.txt properly branches out for
conditional dependencies. Rather we should leave the global scope untouched.

Pull Request resolved: https://github.com/pytorch/audio/pull/2282

Reviewed By: hwangjeff

Differential Revision: D35059838

Pulled By: mthrok

fbshipit-source-id: ed3557eaa9a669e4466d64893beab5089eca78b8