ReduceFrontSumOp: striped Axpby
ReduceFrontSumOp: striped Axpby

Summary: Instead of calling math::Axpby in a loop, we can do it in one kernel much more efficiently.

Reviewed By: asaadaldien, jamesr66a

Differential Revision: D4935893

fbshipit-source-id: 33497784604d1779723d578ea5400e87803851f0

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