node unit testing reports failure in cudnn operation

test-nodes reports

C:/gits/dynet/tests/test-nodes.cc(1674): error: in "node_test/conv2d_valid_singlefilter_gradient": check check_grad(mod, z, 0) has failed

@redpony: I've narrowed it down to being caused by commit 84ca496ab41077289a502589c116c7be81a1bfb7 (which I see caused some other testing errors that you needed to fix). This one probably wasn't caught because testing is not being done on the cudnn code by default (?). I wasn't sure what it was about the changes you made that fixed the unit testing, can you suggest a change for the cudnn failure (I can test it out for you if that's more convenient for you).

Note that you will be unable to compile the cudnn code until PR #1149 is accepted.

Start with tests/test-nodes.cc at line 1674 and the node_test/conv2d_valid_singlefilter_gradient failure, then review commit 84ca496ab41077289a502589c116c7be81a1bfb7. Check PR #1149 first, since the issue says cuDNN cannot compile until it is accepted. Done means the cuDNN unit test no longer reports the gradient failure.

Written by the indexing model from the issue text.