message: remove tracer check
diff: @@ -493,7 +493,7 @@ class DAG(object):
  is_thread_op: bool,
  channel_size,
  build_dag_each_worker: [bool, None],
  - tracer: [dict, None],
  + tracer,
  channel_recv_frist_arrive):
  self._request_name = request_name
  self._response_op = response_op
  