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Summary:
Currently, the doc build job uses `pytorch/manylinux-cuda100` as base environment image.
This PR changes that with `python:3.X`.

The problem with the previous one is
- The image is unnecessarily huge with tools not needed for build doc. (+3GB)
- No easy way to install ffmpeg>=4.1.

https://518849-90321822-gh.circle-artifacts.com/0/docs/index.html

Pull Request resolved: https://github.com/pytorch/audio/pull/2151

Reviewed By: carolineechen

Differential Revision: D33585043

Pulled By: mthrok

fbshipit-source-id: d6d2f6ab33511b8f5c7ca358bc6545e253c1b752