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Summary:
Adds documentation that introduces forthcoming I/O backend revision and provides enablement directions for the current release.

Doc pages:
https://output.circle-artifacts.com/output/job/9c0e5a49-eaf4-404c-b910-ca1b18bb289b/artifacts/0/docs/torchaudio.html

Pull Request resolved: https://github.com/pytorch/audio/pull/3147

Reviewed By: mthrok

Differential Revision: D43824019

Pulled By: hwangjeff

fbshipit-source-id: ad21d60c7e8f69f64859c56a8ca75735ddc22e40

Summary:
This commit updates the documentation configuration so that if an API (function or class) is used in tutorials, then it automatically add the links to the tutorials.

It also adds `py:func:` so that it's easy to jump from tutorials to API reference.

Note: the use of `py:func:` is not required to be recognized by Shpinx-gallery.

* https://482162-90321822-gh.circle-artifacts.com/0/docs/transforms.html#feature-extractions

<img width="776" alt="Screen Shot 2021-12-24 at 12 41 43 PM" src="https://user-images.githubusercontent.com/855818/147367407-cd86f114-7177-426a-b5ee-a25af17ae476.png">

* https://482162-90321822-gh.circle-artifacts.com/0/docs/transforms.html#mvdr

<img width="769" alt="Screen Shot 2021-12-24 at 12 42 31 PM" src="https://user-images.githubusercontent.com/855818/147367422-01fd245f-2f25-4875-a206-910e17ae0161.png">

Pull Request resolved: https://github.com/pytorch/audio/pull/2101

Reviewed By: hwangjeff

Differential Revision: D33311283

Pulled By: mthrok

fbshipit-source-id: e0c124d2a761e0f8d81c3d14c4ffc836ffffe288

- Addresses #549 #638 #786 
- Add `torchaudio` top level module doc
- Separate `torchaudio` top level module doc from `index.html`
- Add `backend` module doc.
- Remove `-> None` from function signature as it adds noise to documentation
- Changed function argument name of `torchaudio.backend.sox_io_backend.save` from `tensor` to `src`, so that it matches with the reset of backends.
- Tweak bunch of docstrings