cmake: allow to build pytorch as a CMake subproject (#110373)
This is a re-attempt of fixing https://github.com/pytorch/pytorch/issues/53980, first submitted in https://github.com/pytorch/pytorch/pull/54978.
Quoting @SpaceIm
```
Fixes https://github.com/pytorch/pytorch/issues/53980
Maybe it would be nice to find why some files are generated in CMAKE_BINARY_DIR instead of CMAKE_CURRENT_BINARY_DIR or Torch_BINARY_DIR or PROJECT_BINARY_DIR, but there is a lot of indirection in the logic of pytorch build files, so I was not able to find where it comes from.
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110373
Approved by: https://github.com/malfet
