Update hardware accelerated video processing tutorial (pytorch#3050)

Summary:
Par pytorch#3040 and pytorch#3041, it turned out Google Colab now has FFmpeg with GPU decoder/encoder preinstalled, and installing FFmpeg manually corrups the environment.

This commit updates the tutorial by extracting and moving the how-to-install part to installation/build section.

closes pytorch#3041
closes pytorch#3040

Pull Request resolved: pytorch#3050

Reviewed By: nateanl

Differential Revision: D43166054

Pulled By: mthrok

fbshipit-source-id: 32667f292a796344d5fcde86e8231e15ad904e58