Replace ReduceDimsOps math::Gemv with CUDA reduction kernel. 5.6x speed up.
Replace ReduceDimsOps math::Gemv with CUDA reduction kernel. 5.6x speed up.

Summary: This reduces runtime from 1.54757 ms/iter -> 0.273687 ms/iter for an 100 parallel reductions each of size 100000.

Reviewed By: akyrola

Differential Revision: D5471324

fbshipit-source-id: 626cabb8249fb4655275648fae2738cb739e1a72

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