langchain/chains/qa_generation/base.py/QAGenerationChain/from_llm
class QAGenerationChain:    def from_llm(
        cls,
        llm: BaseLanguageModel,
        prompt: Optional[BasePromptTemplate] = None,
        **kwargs: Any,
    ) -> QAGenerationChain:
        _prompt = prompt or PROMPT_SELECTOR.get_prompt(llm)
        chain = LLMChain(llm=llm, prompt=_prompt)
        return cls(llm_chain=chain, **kwargs)