fixed Op.__call__ to use the default_output field

gof/op.py

@@ -26,6 +26,9 @@ class Op(object2):
 
     def __call__(self, *inputs):
         node = self.make_node(*inputs)
+        if self.default_output is not None:
+            return node.outputs[self.default_output]
+        else:
             if len(node.outputs) == 1:
                 return node.outputs[0]
             else: