caffe2:libtorch_cuda depends on caffe2:caffe2_gpu (#17729)

caffe2:libtorch_cuda depends on caffe2:caffe2_gpu (#17729)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17729

When doing "import torch" in fbcode, previously the caffe2 cuda kernels weren't loaded because libcaffe2_gpu.so wasn't loaded.
Once you also did "from caffe2.python import workspace", then the cuda kernels were loaded because that triggered a runtime mechanism for loading libcaffe2_gpu.so.

We want the cuda kernels to always be available, so this diff adds a dependency from caffe2:libtorch_cuda to caffe2:caffe2_gpu.

Reviewed By: ezyang

Differential Revision: D14353498

fbshipit-source-id: 76a9fe69f231b308ab40eac393bb216c6fad3658