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Summary:
This commit adds tutorial to enable/use NVDEC with Stream API.

https://output.circle-artifacts.com/output/job/19e66a4b-1819-4804-8834-d38e6c80c4fd/artifacts/0/docs/hw_acceleration_tutorial.html

Because the use of NVDEC requires build / install FFmpeg from source,
this tutorial was authored on Google Colab, tailored to its environment.

The tutorial here is the result of the notebook execution, with
the link to the publicly accessible Google Colab notebook.

Pull Request resolved: https://github.com/pytorch/audio/pull/2393

Reviewed By: hwangjeff

Differential Revision: D36404408

Pulled By: mthrok

fbshipit-source-id: 9c820d3db4d06c5b343ecad0708489125ca06948