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Summary:
* tutorial for streaming API https://541810-90321822-gh.circle-artifacts.com/0/docs/tutorials/streaming_api_tutorial.html
* tutorial for online speech recognition with Emformer RNN-T https://541810-90321822-gh.circle-artifacts.com/0/docs/tutorials/online_asr_tutorial.html

Pull Request resolved: https://github.com/pytorch/audio/pull/2193

Reviewed By: hwangjeff

Differential Revision: D33971312

Pulled By: mthrok

fbshipit-source-id: f9b69114255f15eaf4463ca85b3efb0ba321a95f

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.