fp16: CuDNNSoftmax
fp16: CuDNNSoftmax

Summary:
Was https://github.com/caffe2/caffe2/pull/1151
Closes https://github.com/caffe2/caffe2/pull/1181

Differential Revision: D5794693

Pulled By: akyrola

fbshipit-source-id: c83a98968bb363f612e53a04e8236582be6edd5d

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