improve softmax-with-loss kernels for prob-mode
improve softmax-with-loss kernels for prob-mode

Summary: Yet another diff to improve softmax CUDA kernels. 1) Use CUB for reduction ProbCrossEntropyKernel (was sequential loop); 2) remove unnecessary inner for-loops for two other kernels.

Reviewed By: wickedfoo

Differential Revision: D4953099

fbshipit-source-id: 4a5806d450021eff84e3d7fb0e7020cb5013fd69

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