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Summary:
This commit changes the way doc is pushed.
It ammends instead of adding a new commit.

Currently each commit in gh-pages contain like 100MB of data. gh-pages branch is fetched by default when `git clone`. So the size of torchaudio repo grows significantly.

Pull Request resolved: https://github.com/pytorch/audio/pull/3345

Reviewed By: nateanl

Differential Revision: D46136612

Pulled By: mthrok

fbshipit-source-id: 39479ee5d1a6888254ef50f0db252453d976d183