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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.