Add async support to routing chains (#5373)

# Add async support for (LLM) routing chains

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Add asynchronous LLM calls support for the routing chains. More
specifically:
- Add async `aroute` function (i.e. async version of `route`) to the
`RouterChain` which calls the routing LLM asynchronously
- Implement the async `_acall` for the `LLMRouterChain`
- Implement the async `_acall` function for `MultiRouteChain` which
first calls asynchronously the routing chain with its new `aroute`
function, and then calls asynchronously the relevant destination chain.

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## Who can review?

- @agola11

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langchain/chains/router/base.py

@@ -6,7 +6,11 @@ from typing import Any, Dict, List, Mapping, NamedTuple, Optional
 
 from pydantic import Extra
 
-from langchain.callbacks.manager import CallbackManagerForChainRun, Callbacks
+from langchain.callbacks.manager import (
+    AsyncCallbackManagerForChainRun,
+    CallbackManagerForChainRun,
+    Callbacks,
+)
 from langchain.chains.base import Chain
 
 
@@ -26,6 +30,12 @@ class RouterChain(Chain, ABC):
         result = self(inputs, callbacks=callbacks)
         return Route(result["destination"], result["next_inputs"])
 
+    async def aroute(
+        self, inputs: Dict[str, Any], callbacks: Callbacks = None
+    ) -> Route:
+        result = await self.acall(inputs, callbacks=callbacks)
+        return Route(result["destination"], result["next_inputs"])
+
 
 class MultiRouteChain(Chain):
     """Use a single chain to route an input to one of multiple candidate chains."""
@@ -86,3 +96,32 @@ class MultiRouteChain(Chain):
             raise ValueError(
                 f"Received invalid destination chain name '{route.destination}'"
             )
+
+    async def _acall(
+        self,
+        inputs: Dict[str, Any],
+        run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
+    ) -> Dict[str, Any]:
+        _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
+        callbacks = _run_manager.get_child()
+        route = await self.router_chain.aroute(inputs, callbacks=callbacks)
+
+        _run_manager.on_text(
+            str(route.destination) + ": " + str(route.next_inputs), verbose=self.verbose
+        )
+        if not route.destination:
+            return await self.default_chain.acall(
+                route.next_inputs, callbacks=callbacks
+            )
+        elif route.destination in self.destination_chains:
+            return await self.destination_chains[route.destination].acall(
+                route.next_inputs, callbacks=callbacks
+            )
+        elif self.silent_errors:
+            return await self.default_chain.acall(
+                route.next_inputs, callbacks=callbacks
+            )
+        else:
+            raise ValueError(
+                f"Received invalid destination chain name '{route.destination}'"
+            )

langchain/chains/router/llm_router.py

@@ -6,7 +6,10 @@ from typing import Any, Dict, List, Optional, Type, cast
 from pydantic import root_validator
 
 from langchain.base_language import BaseLanguageModel
-from langchain.callbacks.manager import CallbackManagerForChainRun
+from langchain.callbacks.manager import (
+    AsyncCallbackManagerForChainRun,
+    CallbackManagerForChainRun,
+)
 from langchain.chains import LLMChain
 from langchain.chains.router.base import RouterChain
 from langchain.output_parsers.json import parse_and_check_json_markdown
@@ -58,6 +61,19 @@ class LLMRouterChain(RouterChain):
         )
         return output
 
+    async def _acall(
+        self,
+        inputs: Dict[str, Any],
+        run_manager: Optional[AsyncCallbackManagerForChainRun] = None,
+    ) -> Dict[str, Any]:
+        _run_manager = run_manager or CallbackManagerForChainRun.get_noop_manager()
+        callbacks = _run_manager.get_child()
+        output = cast(
+            Dict[str, Any],
+            await self.llm_chain.apredict_and_parse(callbacks=callbacks, **inputs),
+        )
+        return output
+
     @classmethod
     def from_llm(
         cls, llm: BaseLanguageModel, prompt: BasePromptTemplate, **kwargs: Any