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Summary:
The motivation of generating `artifact.tar.gz` in the `build_docs` job is to easily use it for adding documentation in each stable release. But it is committed into `gh-pages` branch which causes the git repository very huge (see https://github.com/pytorch/audio/issues/2783). This PR removes the tar file from the commit.

Pull Request resolved: https://github.com/pytorch/audio/pull/2786

Reviewed By: carolineechen

Differential Revision: D40591152

Pulled By: nateanl

fbshipit-source-id: 47df60c2ec7bcdcc40e2b6078219b9397e6bfed1