Fixed multi input prompt for MapReduceChain (#4979)

# Fixed multi input prompt for MapReduceChain

Added `kwargs` support for inner chains of `MapReduceChain` via
`from_params` method
Currently the `from_method` method of intialising `MapReduceChain` chain
doesn't work if prompt has multiple inputs. It happens because it uses
`StuffDocumentsChain` and `MapReduceDocumentsChain` underneath, both of
them require specifying `document_variable_name` if `prompt` of their
`llm_chain` has more than one `input`.

With this PR, I have added support for passing their respective `kwargs`
via the `from_params` method.

## Fixes https://github.com/hwchase17/langchain/issues/4752

## Who can review? 
@dev2049 @hwchase17 @agola11

---------

Co-authored-by: imeckr <chandanroutray2012@gmail.com>

docs/modules/chains/index_examples/summarize.ipynb

@@ -21,7 +21,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 1,
+   "execution_count": 2,
    "id": "e9db25f3",
    "metadata": {},
    "outputs": [],
@@ -318,6 +318,141 @@
     "chain({\"input_documents\": docs}, return_only_outputs=True)"
    ]
   },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "id": "b882e209",
+   "metadata": {},
+   "source": [
+    "## The custom `MapReduceChain`\n",
+    "\n",
+    "**Multi input prompt**\n",
+    "\n",
+    "You can also use prompt with multi input. In this example, we will use a MapReduce chain to answer specifc question about our code."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 9,
+   "id": "f7ad9ee2",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from langchain.chains.combine_documents.map_reduce import MapReduceDocumentsChain\n",
+    "from langchain.chains.combine_documents.stuff import StuffDocumentsChain\n",
+    "\n",
+    "map_template_string = \"\"\"Give the following python code information, generate a description that explains what the code does and also mention the time complexity.\n",
+    "Code:\n",
+    "{code}\n",
+    "\n",
+    "Return the the description in the following format:\n",
+    "name of the function: description of the function\n",
+    "\"\"\"\n",
+    "\n",
+    "\n",
+    "reduce_template_string = \"\"\"Give the following following python fuctions name and their descritpion, answer the following question\n",
+    "{code_description}\n",
+    "Question: {question}\n",
+    "Answer:\n",
+    "\"\"\"\n",
+    "\n",
+    "MAP_PROMPT = PromptTemplate(input_variables=[\"code\"], template=map_template_string)\n",
+    "REDUCE_PROMPT = PromptTemplate(input_variables=[\"code_description\", \"question\"], template=reduce_template_string)\n",
+    "\n",
+    "llm = OpenAI()\n",
+    "\n",
+    "map_llm_chain = LLMChain(llm=llm, prompt=MAP_PROMPT)\n",
+    "reduce_llm_chain = LLMChain(llm=llm, prompt=REDUCE_PROMPT)\n",
+    "\n",
+    "generative_result_reduce_chain = StuffDocumentsChain(\n",
+    "    llm_chain=reduce_llm_chain,\n",
+    "    document_variable_name=\"code_description\",\n",
+    ")\n",
+    "\n",
+    "combine_documents = MapReduceDocumentsChain(\n",
+    "    llm_chain=map_llm_chain,\n",
+    "    combine_document_chain=generative_result_reduce_chain,\n",
+    "    document_variable_name=\"code\",\n",
+    ")\n",
+    "\n",
+    "map_reduce = MapReduceChain(\n",
+    "    combine_documents_chain=combine_documents,\n",
+    "    text_splitter=CharacterTextSplitter(separator=\"\\n##\\n\", chunk_size=100, chunk_overlap=0),\n",
+    ")"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 12,
+   "id": "0d4caccb",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "code = \"\"\"\n",
+    "def bubblesort(list):\n",
+    "   for iter_num in range(len(list)-1,0,-1):\n",
+    "      for idx in range(iter_num):\n",
+    "         if list[idx]>list[idx+1]:\n",
+    "            temp = list[idx]\n",
+    "            list[idx] = list[idx+1]\n",
+    "            list[idx+1] = temp\n",
+    "    return list\n",
+    "##\n",
+    "def insertion_sort(InputList):\n",
+    "   for i in range(1, len(InputList)):\n",
+    "      j = i-1\n",
+    "      nxt_element = InputList[i]\n",
+    "   while (InputList[j] > nxt_element) and (j >= 0):\n",
+    "      InputList[j+1] = InputList[j]\n",
+    "      j=j-1\n",
+    "   InputList[j+1] = nxt_element\n",
+    "   return InputList\n",
+    "##\n",
+    "def shellSort(input_list):\n",
+    "   gap = len(input_list) // 2\n",
+    "   while gap > 0:\n",
+    "      for i in range(gap, len(input_list)):\n",
+    "         temp = input_list[i]\n",
+    "         j = i\n",
+    "   while j >= gap and input_list[j - gap] > temp:\n",
+    "      input_list[j] = input_list[j - gap]\n",
+    "      j = j-gap\n",
+    "      input_list[j] = temp\n",
+    "   gap = gap//2\n",
+    "   return input_list\n",
+    "\n",
+    "\"\"\""
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 13,
+   "id": "d5a9a35b",
+   "metadata": {},
+   "outputs": [
+    {
+     "name": "stderr",
+     "output_type": "stream",
+     "text": [
+      "Created a chunk of size 247, which is longer than the specified 100\n",
+      "Created a chunk of size 267, which is longer than the specified 100\n"
+     ]
+    },
+    {
+     "data": {
+      "text/plain": [
+       "'shellSort has a better time complexity than both bubblesort and insertion_sort, as it has a time complexity of O(n^2), while the other two have a time complexity of O(n^2).'"
+      ]
+     },
+     "execution_count": 13,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "map_reduce.run(input_text=code, question=\"Which function has a better time complexity?\")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "id": "f61350f9",
@@ -470,7 +605,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.9.1"
+   "version": "3.8.16"
   },
   "vscode": {
    "interpreter": {

langchain/chains/mapreduce.py

@@ -5,7 +5,7 @@ then combines the results with another one.
 """
 from __future__ import annotations
 
-from typing import Any, Dict, List, Optional
+from typing import Any, Dict, List, Mapping, Optional
 
 from pydantic import Extra
 
@@ -38,15 +38,22 @@ class MapReduceChain(Chain):
         prompt: BasePromptTemplate,
         text_splitter: TextSplitter,
         callbacks: Callbacks = None,
+        combine_chain_kwargs: Optional[Mapping[str, Any]] = None,
+        reduce_chain_kwargs: Optional[Mapping[str, Any]] = None,
         **kwargs: Any,
     ) -> MapReduceChain:
         """Construct a map-reduce chain that uses the chain for map and reduce."""
         llm_chain = LLMChain(llm=llm, prompt=prompt, callbacks=callbacks)
-        reduce_chain = StuffDocumentsChain(llm_chain=llm_chain, callbacks=callbacks)
+        reduce_chain = StuffDocumentsChain(
+            llm_chain=llm_chain,
+            callbacks=callbacks,
+            **(reduce_chain_kwargs if reduce_chain_kwargs else {}),
+        )
         combine_documents_chain = MapReduceDocumentsChain(
             llm_chain=llm_chain,
             combine_document_chain=reduce_chain,
             callbacks=callbacks,
+            **(combine_chain_kwargs if combine_chain_kwargs else {}),
         )
         return cls(
             combine_documents_chain=combine_documents_chain,
@@ -84,9 +91,14 @@ class MapReduceChain(Chain):
     ) -> Dict[str, str]:
         _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
         # Split the larger text into smaller chunks.
-        texts = self.text_splitter.split_text(inputs[self.input_key])
+        doc_text = inputs.pop(self.input_key)
+        texts = self.text_splitter.split_text(doc_text)
         docs = [Document(page_content=text) for text in texts]
+        _inputs: Dict[str, Any] = {
+            **inputs,
+            self.combine_documents_chain.input_key: docs,
+        }
         outputs = self.combine_documents_chain.run(
-            input_documents=docs, callbacks=_run_manager.get_child()
+            _inputs, callbacks=_run_manager.get_child()
         )
         return {self.output_key: outputs}