Remove dead code in whisper ONNX output (#741)

fix whisper output

optimum/exporters/onnx/model_configs.py

@@ -916,11 +916,9 @@ def inputs(self) -> Mapping[str, Mapping[int, str]]:
     def outputs(self) -> Mapping[str, Mapping[int, str]]:
         common_outputs = super().outputs
         if self._behavior is ConfigBehavior.ENCODER:
-            for name, dyanmic_axes in common_outputs.items():
-                if name == "last_hidden_state":
-                    common_outputs[name][1] = f"{common_outputs[name][1]} / 2"
-                else:
-                    common_outputs[name] = dynamic_axes
+            # for whisper, we need to name the second axis as
+            # encoder_sequence_length / 2 as the axis name is used for dummy input generation
+            common_outputs["last_hidden_state"][1] = f"{common_outputs['last_hidden_state'][1]} / 2"
         return common_outputs