add R_op for DisconnectedGrad

theano/gradient.py

@@ -1981,6 +1981,9 @@ class DisconnectedGrad(ViewOp):
     def grad(self, args, g_outs):
         return [disconnected_type() for g_out in g_outs]
 
+    def R_op(self, inputs, eval_points):
+        return [None]
+
     def connection_pattern(self, node):
         return [[False]]