PlaceholderWithDefault needs IR4:

Author: guschmue

tf2onnx/constants.py

@@ -40,5 +40,5 @@
 
 # Mapping opset to IR version.
 OPSET_TO_IR_VERSION = {
-    1: 3, 2: 3, 3: 3, 4: 3, 5: 3, 6: 3, 7: 3, 8: 4, 9: 4, 10: 5, 11: 6, 12: 7
+    1: 3, 2: 3, 3: 3, 4: 3, 5: 3, 6: 3, 7: 4, 8: 4, 9: 4, 10: 5, 11: 6, 12: 7
 }