Split cpu/gpu in caffe2/distributed + some clean up (#20674)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20674
A few targets in caffe2/caffe2/distribute needs to be split too, otherwise won't compile. Also some clean ups and make select_gpu_type to gpu_library_selector
Differential Revision: D15406019
fbshipit-source-id: 6455ab885b248502b48d4c7565597e00fecfd547
