add length assertion

Author: titaiwangms

beginner_source/onnx/onnx_registry_tutorial.py

@@ -128,6 +128,8 @@ def custom_aten_add(input_x, input_y, alpha: float = 1.0):
 assert len(onnx_program.model_proto.graph.node) == 1
 # graph node name is the function name
 assert onnx_program.model_proto.graph.node[0].op_type == "custom_aten_add"
+# the function has three nodes
+assert len(onnx_program.model_proto.functions[0].node) == 3
 # function node domain is empty because we use standard ONNX operators
 assert onnx_program.model_proto.functions[0].node[2].domain == ""
 # function node name is the standard ONNX operator name