Use passed LLM for default chain in MultiPromptChain (#4418)
Currently, MultiPromptChain instantiates a ChatOpenAI LLM instance for
the default chain to use if none of the prompts passed match. This seems
like an error as it means that you can't use your choice of LLM, or
configure how to instantiate the default LLM (e.g. passing in an API key
that isn't in the usual env variable).
