Correct the use of abstractmethod (#80)

* Correct abstractmethod usage

* Update interface

* Specify minimal llama-index version [ignore cache]

* Update examples

knowledgehub/chatbot/simple_respondent.py

@@ -1,5 +1,3 @@
-from theflow import Node
-
 from ..llms import ChatLLM
 from .base import BaseChatBot
 
@@ -7,7 +5,7 @@ from .base import BaseChatBot
 class SimpleRespondentChatbot(BaseChatBot):
     """Simple text respondent chatbot that essentially wraps around a chat LLM"""
 
-    llm: Node[ChatLLM]
+    llm: ChatLLM
 
     def _get_message(self) -> str:
         return self.llm(self.history).text