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The hack introduced in #1969 could break if upstream theme changes its HTML or Javascript.
To prevent our documentation from randomly break, this commit fixes the commit of the them.

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.

* Use `bibtex` for paper citations.
  * add `override.css` for fixing back reference.
  * wav2vec2
  * wav2letter
  * convtasnet
  * deepspeech
  * rnnt-loss
  * griffinlim
* Fix broken references in `filtering`.
* Fix note in soundfile backends.
* Tweak wav2vec2 example.
* Removes unused `pytorch_theme.css`