Add the option to use separate LLMs for GraphCypherQA chain (#9689)

The Graph Chains are different in the way that it uses two LLMChains
instead of one like the retrievalQA chains. Therefore, sometimes you
want to use different LLM to generate the database query and to generate
the final answer.

This feature would make it more convenient to use different LLMs in the
same chain.

I have also renamed the Graph DB QA Chain to Neo4j DB QA Chain in the
documentation only as it is used only for Neo4j. The naming was
ambigious as it was the first graphQA chain added and wasn't sure how do
you want to spin it.

libs/langchain/langchain/chains/graph_qa/cypher.py

@@ -73,15 +73,28 @@ class GraphCypherQAChain(Chain):
     @classmethod
     def from_llm(
         cls,
-        llm: BaseLanguageModel,
+        llm: Optional[BaseLanguageModel] = None,
         *,
         qa_prompt: BasePromptTemplate = CYPHER_QA_PROMPT,
         cypher_prompt: BasePromptTemplate = CYPHER_GENERATION_PROMPT,
+        cypher_llm: Optional[BaseLanguageModel] = None,
+        qa_llm: Optional[BaseLanguageModel] = None,
         **kwargs: Any,
     ) -> GraphCypherQAChain:
         """Initialize from LLM."""
-        qa_chain = LLMChain(llm=llm, prompt=qa_prompt)
-        cypher_generation_chain = LLMChain(llm=llm, prompt=cypher_prompt)
+
+        if not cypher_llm and not llm:
+            raise ValueError("Either `llm` or `cypher_llm` parameters must be provided")
+        if not qa_llm and not llm:
+            raise ValueError("Either `llm` or `qa_llm` parameters must be provided")
+        if cypher_llm and qa_llm and llm:
+            raise ValueError(
+                "You can specify up to two of 'cypher_llm', 'qa_llm'"
+                ", and 'llm', but not all three simultaneously."
+            )
+
+        qa_chain = LLMChain(llm=qa_llm or llm, prompt=qa_prompt)
+        cypher_generation_chain = LLMChain(llm=cypher_llm or llm, prompt=cypher_prompt)
 
         return cls(
             qa_chain=qa_chain,