Fix `main_input_name` in `src/transformers/keras_callbacks.py` (#24916)

fix
Co-authored-by: ydshieh <ydshieh@users.noreply.github.com>

src/transformers/keras_callbacks.py

@@ -193,8 +193,7 @@ class KerasMetricCallback(Callback):
             # This dense conditional recognizes the case where we have an encoder-decoder model, but
             # avoids getting tangled up when we just have a model with a layer called 'encoder'
             if hasattr(self.model, "encoder") and hasattr(self.model.encoder, "main_input_name"):
-                if self.model.encoder.main_input_name != self.model.main_input_name:
-                    main_input_name = self.model.encoder.main_input_name
+                main_input_name = self.model.encoder.main_input_name
             else:
                 main_input_name = getattr(self.model, "main_input_name", "input_ids")