fix(api): add `version` class method for subclasses of `LLMNode`.

`QuestionClassifierNode` and `KnowledgeRetrievalNode` are inherited from
`LLMNode`, so they inherit the `version` method from `LLMNode`.
However, the inheritence relationship is suboptimial and makes code less clear,
creating a tight coupling with `LLMNode`.

Here we explicitly add the `version` to those two classes, improving
readibility and maintainability.

api/core/workflow/nodes/knowledge_retrieval/knowledge_retrieval_node.py

@@ -71,6 +71,10 @@ class KnowledgeRetrievalNode(LLMNode):
     _node_data_cls = KnowledgeRetrievalNodeData  # type: ignore
     _node_type = NodeType.KNOWLEDGE_RETRIEVAL
 
+    @classmethod
+    def version(cls):
+        return "1"
+
     def _run(self) -> NodeRunResult:  # type: ignore
         node_data = cast(KnowledgeRetrievalNodeData, self.node_data)
         # extract variables

api/core/workflow/nodes/question_classifier/question_classifier_node.py

@@ -40,6 +40,10 @@ class QuestionClassifierNode(LLMNode):
     _node_data_cls = QuestionClassifierNodeData  # type: ignore
     _node_type = NodeType.QUESTION_CLASSIFIER
 
+    @classmethod
+    def version(cls):
+        return "1"
+
     def _run(self):
         node_data = cast(QuestionClassifierNodeData, self.node_data)
         variable_pool = self.graph_runtime_state.variable_pool