langchain: Fix Evaluator's _check_evaluation_args (#31910)

Co-authored-by: Mason Daugherty <[email protected]>

Author: cbornet

libs/langchain/langchain/evaluation/schema.py

@@ -120,10 +120,10 @@ def _check_evaluation_args(
             ValueError: If the evaluator requires an input string but none is provided,
                 or if the evaluator requires a reference label but none is provided.
         """
-        if self.requires_input and input is None:
+        if self.requires_input and input_ is None:
             msg = f"{self.__class__.__name__} requires an input string."
             raise ValueError(msg)
-        if input is not None and not self.requires_input:
+        if input_ is not None and not self.requires_input:
             warn(self._skip_input_warning)
         if self.requires_reference and reference is None:
             msg = f"{self.__class__.__name__} requires a reference string."