add GPTAssistantAgent is_termination_msg valid (#1642)

* add GPTAssistantAgent is_termination_msg valid

* make is_termination_msg in GPTAssistantAgent valid and reformatted

* to pass the black format test

Author: yousonnet

autogen/agentchat/contrib/gpt_assistant_agent.py

@@ -145,6 +145,8 @@ def __init__(
         self._openai_threads = {}
         self._unread_index = defaultdict(int)
         self.register_reply(Agent, GPTAssistantAgent._invoke_assistant)
+        self.register_reply(Agent, GPTAssistantAgent.check_termination_and_human_reply)
+        self.register_reply(Agent, GPTAssistantAgent.a_check_termination_and_human_reply)
 
     def _invoke_assistant(
         self,