Summary:
Currently tools/build_pytorch_libs.py looks quite convoluted. This commit reorgnizes cmake related functions to a separate file to make the code clearer.
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This is hopefully helpful for further contribution for better integration with cmake.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21367
Differential Revision: D15636991
Pulled By: soumith
fbshipit-source-id: 44d76e4e77aec0ce33cb32962b6a79a7f82785da
