fix bug

mul input may come from same transpose node

Author: zhijxu-MS

tf2onnx/optimizer/transpose_optimizer.py

@@ -380,7 +380,7 @@ def _mul_handler(self, trans, node):
                 multiplier_input_id = i
                 multiplier_input_node = input_node
 
-        if not multiplier_input_node.is_const():
+        if multiplier_input_node is None or not multiplier_input_node.is_const():
             return False
         multiplier = multiplier_input_node.get_tensor_value(as_list=False)