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Refer to #903 for the overview of planned I/O changes.

* Change the default backend from `"sox"(deprecated)` to `"sox_io"`
* Change the default interface of `"soundfile"` backend to the one identical to `"sox_io"` backend.
* Deprecate torchaudio.USE_SOUNDFILE_LEGACY_INTERFACE
* Update documentations
    * Re-order backends (default first)
    * Update overhaul timeline (removed 0.7.0)
    * Simplify `"soundfile"` backend description

- 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