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Summary:
This commit makes the following changes to the C++ library organization
- Move sox-related feature implementations from `libtorchaudio` to `libtorchaudio_sox`.
- Remove C++ implementation of `is_sox_available` and `is_ffmpeg_available` as it is now sufficient to check the existence of `libtorchaudio_sox` and `libtorchaudio_ffmpeg` to check the availability. This makes `libtorchaudio_sox` and `libtorchaudio_ffmpeg` independent from `libtorchaudio`.
- Move PyBind11-based bindings (`_torchaudio_sox`, `_torchaudio_ffmpeg`) into `torchaudio.lib` so that the built library structure is less cluttered.

Background:
Originally, when the `libsox` was the only C++ extension and `libtorchaudio` was supposed to contain all the C++ code.
The things are different now. We have a bunch of C++ extensions and we need to make the code/build structure more modular.

The new `libtorchaudio_sox` contains the implementations and `_torchaudio_sox` contains the PyBin11-based bindings.

Pull Request resolved: https://github.com/pytorch/audio/pull/2929

Reviewed By: hwangjeff

Differential Revision: D42159594

Pulled By: mthrok

fbshipit-source-id: 1a0fbca9e4143137f6363fc001b2378ce6029aa7