Windows Support

To bring Windows support with mp3 support, we need

• Activate build for wheels and conda package on CircleCI for Windows without SoX, see #394
• Activate SoX tests only when SoX available, see #419
• Fix kaldi_io NameError for Windows in comment.
• Activate CircleCI tests on Windows (e.g. #493)
• Implement per-file-format backend dispatch mechanism.
• Add minimp3 for mp3 support, see comment.
• Activate all tests that depend on mp3 support.
• Activate nightly upload for wheels and conda package, e.g. #385.
• If MinGW is supported by pytorch, then fix #42.

If and only if no backend support mp3 on Windows after the above:

• Compile SoX on Windows
• Activate build for wheels and conda package on CircleCI for Windows with SoX usinc cpp extension, e.g. #394.

Closes #50, closes #219, closes #258.

cc @peterjc123, @chauhang, pytorch/pytorch#24344

[peterjc123]

@vincentqb I saw a post that describes how to compile torchaudio with Sox. Will try that later.

[peterjc123]

@vincentqb It will be easy for conda users because they can simply do conda install -c conda-forge ffmpeg. To make it convenient for other users, we may just distribute the DLLs for them.