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Summary:
The backend dispatcher is implemented in `torchaudio._backend`, while the legacy backend is implemented in `torchaudio.backend`.

The initialization happen in `torchaudio._backend`.
This commit moves it to `torchaudio.__init__`, so that `backend` and `_backend` is more independent.

Pull Request resolved: https://github.com/pytorch/audio/pull/3548

Reviewed By: huangruizhe

Differential Revision: D48219244

Pulled By: mthrok

fbshipit-source-id: e694cb232794f90902a60ee51c7bf11b7f0548a0

Summary:
This commit makes the code defaults to the backend dispatcher by default. Enabling backend dispatcher puts the FFmpeg-based I/O implementation on higher priority (if the corresponding FFmpeg is available), and allows individual function call to specify the backend.

See also https://github.com/pytorch/audio/issues/2950

Pull Request resolved: https://github.com/pytorch/audio/pull/3241

Reviewed By: hwangjeff

Differential Revision: D44709068

Pulled By: mthrok

fbshipit-source-id: 43aac3433f78a681df6669e9ac46e8ecf3beb1be

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

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

As a part of the "sox" backend sunset plan (#903), we add a "soundfile" backend that is compatible with the "sox_io" backend. No new public backend name is added. We provide a switch to change the interface/behavior of "soundfile" backend.

This commit contains;
 - The implementation of the new "soundfile" backend.
 - The flag to switch the behavior of "soundfile" backend. (`torchaudio.USE_SOUNDFILE_LEGACY_INTERFACE`)
 - Test for the new backend and switching mechanism.

The default behavior of "soundfile" backend is not changed. The users who want to opt-in the new "soundfile" interface can do so by `torchaudio.USE_SOUNDFILE_LEGACY_INTERFACE = False` before changing the backend to "soundfile".

In 0.8.0 release, the "soundfile" backend will use this interface by default, and users can still use the legacy one with `torchaudio.USE_SOUNDFILE_LEGACY_INTERFACE = True`. In 0.9.0, the legacy interface is removed and `torchaudio.USE_SOUNDFILE_LEGACY_INTERFACE` flag will be eventually removed.

We have been running unit test with editable installation. (i.e. `python setup.py develop`), with which we missed issues like #842. 

This CC makes installation in CI non-editable, and change test directory structure so that the source code will not shadow the installed version of `torchaudio`. With simple `pytest test`, `pytest` modifies `sys.path` and prepend checked out repository, which shadows the installed version.

To remedy this, the whole test suites has been moved from `./test` to `./test/torchaudio_unittest`. This adds nice module structure to our test code and we can do absolute import in each test module, which makes it possible again to run test with `python -m unittest torchaudio_unittest/XXX.py`

This change does not affect the regular development process (`python setup.py develop` && `pytest test`)

This is a part of PRs to add new "sox_io" backend #726, and depends on #718.

This PR adds `info` function to "sox_io" backend, which allows users to fetch some metadata of an audio file. 
At this moment, the information retrieved are;

 - Number of samples in the audio file
 - Sampling rate
 - Number of channels

* Make tests backend aware by introducing TorchaudioTestCase and reset backend for each TestCase.

* Set backends for the test cases that require specific backend.

* Refactor backend switching

1. Do not rely on global variables for backend switch
   So that load/save/info/load_wav functions will be torchscript-able
2. Add no_backend module to for the case there is no backend module available
   [bonus] This allows the whole codebase importable on systems that do not have torchaudio C++ extension nor soundfile.