Check output_names attr as well.

Signed-off-by: Jay Zhang <[email protected]>

tf2onnx/convert.py

@@ -328,7 +328,8 @@ def _rename_duplicate_keras_model_names(model):
     IMPORTANT: model may be edited. Assign model.output_names to old_out_names to restore.
     """
     old_out_names = None
-    if model.output_names and len(set(model.output_names)) != len(model.output_names):
+    if hasattr(model, "output_names") and model.output_names \
+        and len(set(model.output_names)) != len(model.output_names):
         # In very rare cases, keras has a bug where it will give multiple outputs the same name
         # We must edit the model or the TF trace will fail
         old_out_names = model.output_names