[cmake] Use PROJECT_SOURCE_DIR instead of CMAKE_* (#41387)
Summary:
Add support for including pytorch via an add_subdirectory()
This requires using PROJECT_* instead of CMAKE_* which refer to
the top-most project including pytorch.
TEST=add_subdirectory() into a pytorch checkout and build.
There are still some hardcoded references to TORCH_SRC_DIR, I will
fix in a follow on commit. For now you can create a symlink to
<pytorch>/torch/ in your project.
Change-Id: Ic2a8aec3b08f64e2c23d9e79db83f14a0a896abc
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41387
Reviewed By: zhangguanheng66
Differential Revision: D22539944
Pulled By: ezyang
fbshipit-source-id: b7e9631021938255f0a6ea897a7abb061759093d
