Combine documents by doing a first pass and then refining on more documents.
This algorithm first calls initial_llm_chain on the first document, passing
that first document in with the variable name document_variable_name, and
produces a new variable with the variable name initial_response_name.
Then, it loops over every remaining document. This is called the "refine" step.
It calls refine_llm_chain,
passing in that document with the variable name document_variable_name
as well as the previous response with the variable name initial_response_name.