Fix typo in Layer.add_loss. (keras-team#7657)

Author: hgaiser

keras/engine/topology.py

@@ -1098,7 +1098,7 @@ def add_loss(self, losses, inputs=None):
         if hasattr(self, '_losses'):
             self._losses += losses
         # Update self._per_input_updates
-        if isinstance(input, list) and inputs == []:
+        if isinstance(inputs, list) and inputs == []:
             inputs = None
         if inputs is not None:
             inputs_hash = _object_list_uid(inputs)