Theano/Theano — pull request #877 by goodfeli (2012-08-20 21:38:43+00:00)
Title: Make unimplemented images2neibs gradient handled the same as other unimplemented grads

images2neibs was raising an exception saying that its gradient was unimplemented. This is not how other ops with unimplemented grads are treated--they just return None--and it causes problems where no gradients can be computed in a graph that contains this op anywhere, because of the way T.grad is implemented. This pull request changes it to issue a warning and return None.