Permit none key in chain mapper (#8696)

libs/langchain/langchain/smith/evaluation/string_run_evaluator.py

@@ -126,9 +126,12 @@ class ChainStringRunMapper(StringRunMapper):
     """Extract items to evaluate from the run object from a chain."""
 
     input_key: Optional[str] = None
-    """The key from the model Run's inputs to use as the eval input."""
+    """The key from the model Run's inputs to use as the eval input.
+    If not provided, will use the only input key or raise an error if there are multiple.
+    """
     prediction_key: Optional[str] = None
-    """The key from the model Run's outputs to use as the eval prediction."""
+    """The key from the model Run's outputs to use as the eval prediction.
+    If not provided, will use the only output key or raise an error if there are multiple."""
 
     def _get_key(self, source: Dict, key: Optional[str], which: str) -> str:
         if key is not None:
@@ -145,11 +148,9 @@ class ChainStringRunMapper(StringRunMapper):
         """Maps the Run to a dictionary."""
         if not run.outputs:
             raise ValueError(f"Run {run.id} has no outputs to evaluate.")
-        if run.run_type != "chain":
-            raise ValueError("Chain RunMapper only supports Chain runs.")
-        if self.input_key not in run.inputs:
+        if self.input_key is not None and self.input_key not in run.inputs:
             raise ValueError(f"Run {run.id} does not have input key {self.input_key}.")
-        elif self.prediction_key not in run.outputs:
+        elif self.prediction_key is not None and self.prediction_key not in run.outputs:
             raise ValueError(
                 f"Run {run.id} does not have prediction key {self.prediction_key}."
             )