when BUILD_SPLIT_CUDA=ON, create dummy torch_cuda (#52305)
Summary:
Makes dummy torch_cuda target to maintain better backwards compatibility.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52305
Test Plan:
Run `export BUILD_SPLIT_CUDA=ON && python setup.develop`.
When it's done building, run `ls -lah` within `build/lib` to check that `libtorch_cuda.so` exists and is the same size as `libtorch.so`.
Reviewed By: walterddr
Differential Revision: D26463915
Pulled By: janeyx99
fbshipit-source-id: 2b4cb8ee49bd75e11dc89d94b5956917b1800df1
