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.
I don't see what's wrong with three yields.  But I could change it to `for subsample in (1, 1), (1, 2), (2, 2):`. It is never used separately anyway.