I wanted to setup a chain as follows:

reduce_prompt = hub.pull("rlm/reduce-prompt")
reduce_chain = reduce_prompt | llm_model
combine_documents_chain = StuffDocumentsChain(
  llm_chain=reduce_chain, 
  document_variable_name="doc_summaries")

It gave me an error AttributeError: 'RunnableSequence' object has no attribute 'prompt'

I am not sure what it means. If I change the runnable line reduce_chain = reduce_prompt | llm_model to reduce_chain = LLMChain(prompt = reduce_prompt, llm=llm_model) such that the full code :

reduce_prompt = hub.pull("rlm/reduce-prompt")
reduce_chain = LLMChain(prompt = reduce_prompt, llm=llm_model)
combine_documents_chain = StuffDocumentsChain(
  llm_chain=reduce_chain, 
  document_variable_name="doc_summaries")

my code would run without errors. Could you help explain what went wrong in the original code?