Ops rewritten:

 - GpuDnnConvDesc
 - GpuDnnPool
 - GpuDnnPoolGrad
 - GpuDnnConv
 - GpuDnnConvGradW
 - GpuDnnConvGradI
 - GpuDnnBatchNormInference
 - GpuDnnBatchNormGrad

cuDNN v6 integration:

 - Support MAX DETERMINISTIC algorithm for GpuDnnPool with cuDNN v6.
 - Update pooling tests for DNN module so that they use the right available algorithms depending on runtime cuDNN version.
 - Allow CPU Pool and PoolGrad ops to use MAX_DETERMINISTIC algo when cuDNN v6 is used with GPU counterparts.
 - Encapsulate cuDNN constants used in DNN module, to help choose the right cuDNN definitions depending on the runtime cuDNN version.
   Currently supported cuDNN versions: v5.1, v6.0.