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Summary:
Part of https://github.com/pytorch/audio/issues/2072 -- splitting up PR for easier review

This PR adds Python decoder API and basic README

Pull Request resolved: https://github.com/pytorch/audio/pull/2089

Reviewed By: mthrok

Differential Revision: D33299818

Pulled By: carolineechen

fbshipit-source-id: 778ec3692331e95258d3734f0d4ab60b6618ddbc