* Current cudnn implementations use only
a single dimension for the ngraph tensor data (width).
In this case the tensor format should be set to

CUDNN_TENSOR_NCHW

so that adjacent memory accesses are coalesced (stride=1 for width).

* * Added some kernel emitter helpers that are reused often.
* Renamed EmitElementwise -> emit_elementwise to match emit<T>.
* op::Sum now handles trivial case of dim(input_tensor) = dim(output_tensor)
  by performing a memcpy as no axes are reduced.

*   Added general case for Nd descriptors which is used when the tensor
  has more than 4 dimensions. Currently a naive reduce is performed,
  in the future a coordinate transformation could be performed to
  improve the memory layout for the reduction.

* Switched to codegen::CodeWriter::block_begin/end.
It appears that CodeWriter::block_begin/end is not frequently used for emitters (in cpu and gpu transformers)
because a block comment is often desired. To this end I added prefix/suffix default parameters to CodeWriter::block_begin/end
so that this functionality is captured.