data_parallel_model: NCCLBroadcast root fix
data_parallel_model: NCCLBroadcast root fix

Summary:
The root is the root _rank_ and not the root _device_. Thus we always
use root=0, regardless of the devices used.

https://github.com/NVIDIA/nccl/blob/v1.3.0-1/src/broadcast.cu#L75

/cc slayton58
Closes https://github.com/caffe2/caffe2/pull/872

Differential Revision: D5329564

Pulled By: akyrola

fbshipit-source-id: 5a34be30c1a0046a74f28437cb08333c1fb46098

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