Fix load map reduce documents chain (#7915)

This PR updates _load_reduce_documents_chain to handle
`reduce_documents_chain` and `combine_documents_chain` config

Please review @hwchase17, @baskaryan

Co-authored-by: Harrison Chase <hw.chase.17@gmail.com>

libs/langchain/langchain/chains/loading.py

@@ -114,17 +114,55 @@ def _load_map_reduce_documents_chain(
     if not isinstance(llm_chain, LLMChain):
         raise ValueError(f"Expected LLMChain, got {llm_chain}")
 
-    if "combine_document_chain" in config:
+    if "reduce_documents_chain" in config:
+        reduce_documents_chain = load_chain_from_config(
+            config.pop("reduce_documents_chain")
+        )
+    elif "reduce_documents_chain_path" in config:
+        reduce_documents_chain = load_chain(config.pop("reduce_documents_chain_path"))
+    else:
+        reduce_documents_chain = _load_reduce_documents_chain(config)
+
+    return MapReduceDocumentsChain(
+        llm_chain=llm_chain,
+        reduce_documents_chain=reduce_documents_chain,
+        **config,
+    )
+
+
+def _load_reduce_documents_chain(config: dict, **kwargs: Any) -> ReduceDocumentsChain:
+    combine_documents_chain = None
+    collapse_documents_chain = None
+
+    if "combine_documents_chain" in config:
+        combine_document_chain_config = config.pop("combine_documents_chain")
+        combine_documents_chain = load_chain_from_config(combine_document_chain_config)
+    elif "combine_document_chain" in config:
         combine_document_chain_config = config.pop("combine_document_chain")
         combine_documents_chain = load_chain_from_config(combine_document_chain_config)
+    elif "combine_documents_chain_path" in config:
+        combine_documents_chain = load_chain(config.pop("combine_documents_chain_path"))
     elif "combine_document_chain_path" in config:
         combine_documents_chain = load_chain(config.pop("combine_document_chain_path"))
     else:
         raise ValueError(
-            "One of `combine_document_chain` or "
-            "`combine_document_chain_path` must be present."
+            "One of `combine_documents_chain` or "
+            "`combine_documents_chain_path` must be present."
+        )
+
+    if "collapse_documents_chain" in config:
+        collapse_document_chain_config = config.pop("collapse_documents_chain")
+        if collapse_document_chain_config is None:
+            collapse_documents_chain = None
+        else:
+            collapse_documents_chain = load_chain_from_config(
+                collapse_document_chain_config
+            )
+    elif "collapse_documents_chain_path" in config:
+        collapse_documents_chain = load_chain(
+            config.pop("collapse_documents_chain_path")
         )
-    if "collapse_document_chain" in config:
+    elif "collapse_document_chain" in config:
         collapse_document_chain_config = config.pop("collapse_document_chain")
         if collapse_document_chain_config is None:
             collapse_documents_chain = None
@@ -136,15 +174,10 @@ def _load_map_reduce_documents_chain(
         collapse_documents_chain = load_chain(
             config.pop("collapse_document_chain_path")
         )
-    else:
-        collapse_documents_chain = None
-    reduce_documents_chain = ReduceDocumentsChain(
+
+    return ReduceDocumentsChain(
         combine_documents_chain=combine_documents_chain,
         collapse_documents_chain=collapse_documents_chain,
-    )
-    return MapReduceDocumentsChain(
-        llm_chain=llm_chain,
-        reduce_documents_chain=reduce_documents_chain,
         **config,
     )
 
@@ -497,6 +530,7 @@ type_to_loader_dict = {
     "qa_with_sources_chain": _load_qa_with_sources_chain,
     "stuff_documents_chain": _load_stuff_documents_chain,
     "map_reduce_documents_chain": _load_map_reduce_documents_chain,
+    "reduce_documents_chain": _load_reduce_documents_chain,
     "map_rerank_documents_chain": _load_map_rerank_documents_chain,
     "refine_documents_chain": _load_refine_documents_chain,
     "sql_database_chain": _load_sql_database_chain,