* Refactored the cudnn reduce kernel to use the nGraph Shape -> cudnnTensorDescriptor cudnn helpers that the other kernels use.

* Added cacheing to cudnn reduce op.

* Adding back hashing call before returning primitive index to op::Reduce (bug fix).

* [cuDNN:Part 2] Descriptor Creation/Destruction refactoring (#969)

* Added a cuDNN descriptor factory which manages the construction and destruction of cuDNN descriptors.
It correctly calls Create/Destroy based on the cuDNN descriptor type. Previously the Destroy functions were not being called.

* Removed commented code and changed class to struct on cudnn_descriptor.

* Added comments and updated a few variable names.

* Clang compiled cuDNN kernels (those not part of CUDNNEmitter)
now use the CUDNNDescriptor factory.