Move `torch/version.py` generation to cmake (#44577)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44577
I would like to to move this to cmake so that I can depend on it
happening from other parts of the build.
This PR pulls out the logic for determining the version string and
writing the version file into its own module. `setup.py` still receives
the version string and uses it as before, but now the code for writing
out `torch/version.py` lives in a custom command in torch/CMakeLists.txt
I noticed a small inconsistency in how version info is populated.
`TORCH_BUILD_VERSION` is populated from `setup.py` at configuration
time, while `torch/version.py` is written at build time. So if, e.g. you
configured cmake on a certain git rev, then built it in on another, the
two versions would be inconsistent.
This does not appear to matter, so I opted to preserve the existing
behavior.
Test Plan: Imported from OSS
Reviewed By: bertmaher
Differential Revision: D23734781
Pulled By: suo
fbshipit-source-id: 4002c9ec8058503dc0550f8eece2256bc98c03a4
