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Summary:
This commit add the step to build FFmpeg with GPU decoder in build_doc job so that we can use GPU decoder/encoder in documentations.

Pull Request resolved: https://github.com/pytorch/audio/pull/3045

Reviewed By: nateanl

Differential Revision: D45965739

Pulled By: mthrok

fbshipit-source-id: c167eb3ef347860a51efa906068fa2daa556f017