langchain[patch]: add aprep_output method to langchain/chains/base.py (#20748)

## Description

Add `aprep_output` method to `langchain/chains/base.py`. Some downstream
`ChatMessageHistory` objects that use async connections require an async
way to append to the context.

It turned out that `ainvoke()` was calling `prep_output` which is
synchronous.

---------

Co-authored-by: Bagatur <22008038+baskaryan@users.noreply.github.com>

libs/langchain/langchain/chains/base.py

@@ -204,7 +204,7 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
                 if new_arg_supported
                 else await self._acall(inputs)
             )
-            final_outputs: Dict[str, Any] = self.prep_outputs(
+            final_outputs: Dict[str, Any] = await self.aprep_outputs(
                 inputs, outputs, return_only_outputs
             )
         except BaseException as e:
@@ -458,6 +458,32 @@ class Chain(RunnableSerializable[Dict[str, Any], Dict[str, Any]], ABC):
         else:
             return {**inputs, **outputs}
 
+    async def aprep_outputs(
+        self,
+        inputs: Dict[str, str],
+        outputs: Dict[str, str],
+        return_only_outputs: bool = False,
+    ) -> Dict[str, str]:
+        """Validate and prepare chain outputs, and save info about this run to memory.
+
+        Args:
+            inputs: Dictionary of chain inputs, including any inputs added by chain
+                memory.
+            outputs: Dictionary of initial chain outputs.
+            return_only_outputs: Whether to only return the chain outputs. If False,
+                inputs are also added to the final outputs.
+
+        Returns:
+            A dict of the final chain outputs.
+        """
+        self._validate_outputs(outputs)
+        if self.memory is not None:
+            await self.memory.asave_context(inputs, outputs)
+        if return_only_outputs:
+            return outputs
+        else:
+            return {**inputs, **outputs}
+
     def prep_inputs(self, inputs: Union[Dict[str, Any], Any]) -> Dict[str, str]:
         """Prepare chain inputs, including adding inputs from memory.