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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.