GitLab

Summary:
Pull Request resolved: https://github.com/pytorch/audio/pull/2096

run: `arc lint --apply-patches --paths-cmd 'hg files -I "./**/*.py"'`

Reviewed By: mthrok

Differential Revision: D33297351

fbshipit-source-id: 7bf5956edf0717c5ca90219f72414ff4eeaf5aa8

* Refactor tutorial organization

* Merge tutorial subdirectoris under to examples/gallery/tutorials
* Do not use index.rst generated by Sphinx-gallery
* Instead use flat structure so that all the tutorials are listed in left menu
* Use `_assets` dir for artifacts of tutorials

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.

Resolve the following warnings when `make clean html`.

```
parsing bibtex file /torchaudio/docs/source/refs.bib... parsed 26 entries
loading intersphinx inventory from https://docs.python.org/objects.inv...
loading intersphinx inventory from https://docs.scipy.org/doc/numpy/objects.inv...
loading intersphinx inventory from https://pytorch.org/docs/stable/objects.inv...
intersphinx inventory has moved: https://docs.python.org/objects.inv -> https://docs.python.org/3/objects.inv
intersphinx inventory has moved: https://docs.scipy.org/doc/numpy/objects.inv -> https://numpy.org/doc/stable/objects.inv
```

* 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`