Expand on the comment.

theano/sandbox/gpuarray/opt.py

@@ -807,7 +807,7 @@ theano.tensor.nnet.conv2d()
             AbstractConv2d_gradInputs])
 def local_lift_abstractconv2d(node, context_name):
     if isinstance(node.outputs[0].type, GpuArrayType):
-        # Don't handle this node here.
+        # Don't handle this node here, it's already on the GPU.
         return
     inps = list(node.inputs)
     inps[0] = as_gpuarray_variable(node.inputs[0],