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Summary:
Part of https://github.com/pytorch/audio/issues/1986. Splitting the PR for easier review.

Add `Streamer` class that bundles `StreamProcessor` and handle input.
For the overall architecture, see https://github.com/mthrok/audio/blob/ffmpeg/torchaudio/csrc/ffmpeg/README.md.

Note: Without a change to build process, the code added here won't be compiled. The build process will be updated later.
Needs to be imported after https://github.com/pytorch/audio/issues/2045.

Pull Request resolved: https://github.com/pytorch/audio/pull/2046

Reviewed By: carolineechen

Differential Revision: D33299863

Pulled By: mthrok

fbshipit-source-id: 6470cbe061057c8cb970ce7bb5692be04efb5fe9