add gpu pyserving part

python/paddle_serving_server/pyserver.py

@@ -1099,7 +1099,7 @@ class PyServer(object):
                         processed_op.add(other_op.name)
         output_channel = Channel(self._manager, name=channel_name_gen.next())
         channels.append(output_channel)
-        last_op = dag_views[-1][0]
+        last_op = dag_views[-1][0]  # TODO: fix it
         last_op.add_output_channel(output_channel)
 
         self._actual_ops = virtual_ops