Data parallel model: Disable NCCL by default to hopefully reduce deadlocks
Data parallel model: Disable NCCL by default to hopefully reduce deadlocks

Summary: Make NCCL optional in data_parallel_model due to continuing reliablity (deadlock) issues.

Reviewed By: pietern

Differential Revision: D4988950

fbshipit-source-id: 8a2192f01b5f3c0e847137cd37aefc69e553a56f

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