Add support for subsampling and gradient to GpuDnnConv

This adds a grad() method to GpuDnnConv and adds support for subsampling.  It also allows choosing between convolution and cross-correlation mode (however cross-correlation mode has not been tested).

I've also made a base class that create the cudnn handle to make other ops easier to write.
Let's wait until I have some code that works, then I'll revisit atol.