use distutils to discover msvc compiler paths (#16540)
Summary:
This simplifies the process for building on windows, since users no longer have to find and run the vcvarsall.bat file.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16540
Differential Revision: D13893596
Pulled By: zdevito
fbshipit-source-id: 79b7ad55c3251b3f573fd8464931138f8a52dd1d
