Use Gloo ops in data_parallel_model
Use Gloo ops in data_parallel_model

Summary:
No longer need GPU to CPU copies. The allreduce operator no longer
uses 'local allreduce - global allreduce - local broadcast' sequence
when Gloo is used, but passes all input blobs directly.

Depends on D4708860.

Differential Revision: D4709897

fbshipit-source-id: 4d745d5d8bac9c2fcca081dd5d812c902808c3b6

Yangqing/caffe2,Yangqing/caffe2,xzturn/caffe2,xzturn/caffe2,Yangqing/caffe2,pietern/caffe2,caffe2/caffe2,davinwang/caffe2,davinwang/caffe2,bwasti/caffe2,pietern/caffe2,bwasti/caffe2,davinwang/caffe2,bwasti/caffe2,davinwang/caffe2,sf-wind/caffe2,pietern/caffe2,pietern/caffe2,Yangqing/caffe2,sf-wind/caffe2,sf-wind/caffe2,bwasti/caffe2,pietern/caffe2,davinwang/caffe2,xzturn/caffe2,sf-wind/caffe2,xzturn/caffe2,Yangqing/caffe2,sf-wind/caffe2,bwasti/caffe2,xzturn/caffe2