DeviceInference function for NCCLAllreduce
DeviceInference function for NCCLAllreduce

Summary: Not sure it is correct in general, but it works as long as we have one blob per GPU.

Reviewed By: harouwu

Differential Revision: D5671891

fbshipit-source-id: 739475101e9b509bc521e268c5b308faa36800e7

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