Convnet benchmark cudnn_ws
Convnet benchmark cudnn_ws

Summary:
- Do not set default for cudnn_ws. Will use the default set by cuDNN ops.
- Do not use cudnn_ws for MLP.
- Do not run the benchmark if the required args are not set. Previously tried to run and errors out.
Closes https://github.com/caffe2/caffe2/pull/177

Differential Revision: D4633143

Pulled By: Yangqing

fbshipit-source-id: e89a7d01984e599d92a330d0ee4ba106feba65b8

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