Add `torch.utils.cmake_prefix_path` pointing to `share/cmake` folder (#38559)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/38559
Test Plan: Make sure that `cmake path/to/CMakeLists.txt -DCMAKE_PREFIX_PATH=`python -c 'import torch;print(torch.utils.cmake_prefix_path)'` succeeds for CMake projects which depends on Torch package
Differential Revision: D21644066
Pulled By: malfet
fbshipit-source-id: c8e3cb2cbd7f969fadea6a3bccc41c4edb3ec546
