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Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/3402

This is a second attempt of https://github.com/pytorch/audio/pull/3353.

The basic logic to enable dlopen for FFmpeg libraries are same.
It uses `at::DynamicLibrary`, which allows to compile torchaudio without
linking FFmpeg libraries.

This time, the option to enable this feature DLOPEN_FFMPEG has been added,
so that users have a way to disable this feature and keep using build-time
linking.

Please refer to stub.h for more technical detail.

Differential Revision: D46403783

fbshipit-source-id: ca3db57ff6bdc50c8c225d22f12f3e76c6dc3f16