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This PR moves the code related to PyBind11 to the dedicated directory `torchaudio/csrc/pybind`.

Before, features related to PyBind11 (I/O for file-like object) was implemented in `torchaudio/csrc/sox` and the binding was defined in `torchaudio/csrc/pybind.cpp`. We used macro definition `TORCH_API_INCLUDE_EXTENSION_H` to turn on/off the feature, in addition to including/excluding `torchaudio/csrc/pybind.cpp` in the list of compiled source.

In the previous manner, in C++ example, one had to rebuild libtorchaudio separately, but by splitting them completely at compile time, it should conceptually possible to distribute libtorchaudio within torchaudio Python package and reuse it for C++ example.