Implemented grad for cudnn softmax.
Incorporated reviewer comments.
Added optimization that converts SoftmaxGrad to DnnSoftmaxGrad and associated test to make sure it is applied correctly.
Added test to ensure SoftmaxGrad to DnnSoftmaxGrad is not applied when cudnn is excluded from optimizations.
Incorporated reviewer comment.
Changed variable from T.vector to T.fvector since the data will only be on a GPU.
Merge pull request #2250 from daemonmaker/cudnn

Implemented grad for cudnn softmax.