Unify libtorch and libcaffe2 (#17783)
Summary:
This PR is an intermediate step toward the ultimate goal of eliminating "caffe2" in favor of "torch". This PR moves all of the files that had constituted "libtorch.so" into the "libcaffe2.so" library, and wraps "libcaffe2.so" with a shell library named "libtorch.so". This means that, for now, `caffe2/CMakeLists.txt` becomes a lot bigger, and `torch/CMakeLists.txt` becomes smaller.
The torch Python bindings (`torch_python.so`) still remain in `torch/CMakeLists.txt`.
The follow-up to this PR will rename references to `caffe2` to `torch`, and flatten the shell into one library.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17783
Differential Revision: D15284178
Pulled By: kostmo
fbshipit-source-id: a08387d735ae20652527ced4e69fd75b8ff88b05
