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With the introduction of TTS tutorial (#1973), it takes more than couple of minutes
to build documentation. This commit makes the doc build process defaults to
not build tutorials.

To build tutorials one can use environment variable `BUILD_GALLERY=1`,
and set `GALLERY_PATTERN=...` to filter the tutorials to build.

This `GALLERY_PATTERN` is same approach as in `tutorials` repo.

https://github.com/pytorch/tutorials/blob/cbf2238df0e78d84c15bd94288966d2f4b2e83ae/conf.py#L75-L83

Also this commit dynamically parse the subdirectory of `examples/gallery` so that when a new category of examples are added, it will automatically parsed.

* Import torchaudio #1034 70f429a4

Summary: Import torchaudio #1027 0cf4b8a9

Reviewed By: vincentqb, cpuhrsch

Differential Revision: D24958707

fbshipit-source-id: d06dd6b59197cc2c16bec5a9012cbf33a172b6b3

* Import torchaudio #1066 4406a6bb

Summary: Import up to #1066

Reviewed By: cpuhrsch

Differential Revision: D25373068

fbshipit-source-id: 890d36a25259b93428b3037c3123ff5a2cacfa04