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This PR
 - Changes the location of C++ codes from
     - from `torchaudio/torch_sox.cpp` to `torchaudio/csrc/sox.cpp`
     - from `torchaudio/torch_sox.h` to `torchaudio/csrc/sox.h`
 - Changes the location where the resulting library is placed
     - from `_torch_sox.so` (outside of `torchaudio` module)
        to `torchaudio/_torchaudio.so`.