Add BackConvOp operator, to help compute the gradient of a ConvOp when

dx, dy != 1, 1

Add BackConvOp operator, to help compute the gradient of a ConvOp when
b2f48f9c · Pascal Lamblin · d57899df · b2f48f9c
--- a/theano/sandbox/back_conv.py
+++ b/theano/sandbox/back_conv.py