Remove cuda from autograd profiler (#15898)

Remove cuda from autograd profiler (#15898)

Summary:
This puts stubs in the autograd profiler for the use of cuda APIs allowing the cuda parts of libtorch to be linked separately from the CPU parts.

This also edits the buck build.

Previous:

For GPU builds:
_C -> csrc -> caffe2
For CPU builds:
_C -> csrc-cpu -> caffe2

Now:
GPU:
_C -> libtorch_cuda -> (libtorch -> caffe2, for CPU)

Pull Request resolved: https://github.com/pytorch/pytorch/pull/15898

Reviewed By: ailzhang

Differential Revision: D13617991

Pulled By: zdevito

fbshipit-source-id: 6d84a50bb356a54b4217f93219902755601b00e1