langchain

ed37fbae - for ChatVectorDBChain, add top_k_docs_for_context to allow control how many chunks of context will be retrieved (#1155)

for ChatVectorDBChain, add top_k_docs_for_context to allow control how many chunks of context will be retrieved (#1155) given that we allow user define chunk size, think it would be useful for user to define how many chunks of context will be retrieved.