Bug Report

Describe the bug

The version converter fails on the attached model (and not becase the adapter is missing)

System information

• OS Platform and Distribution: Ubuntu 18.04
• ONNX version: 1.7 release commit
• Python version: 3.6.9
• GCC/Compiler version: 7.5.0
• CMake version: 3.17.3
• Protobuf version: 3.12

Reproduction instructions

import onnx
from onnx import version_converter, helper

# Preprocessing: load the model to be converted.
model_path = "path/to/shared-bn-arrays-v11.onnx"
original_model = onnx.load(model_path)

converted_model = version_converter.convert_version(original_model, 12)
print("done!")

The script doesn’t get to the final print, but fails with

Traceback (most recent call last):
  File "script.py", line 8, in <module>
    converted_model = version_converter.convert_version(original_model, 12)
  File "/usr/local/lib/python3.6/dist-packages/onnx-1.7.0-py3.6-linux-x86_64.egg/onnx/version_converter.py", line 166, in convert_version
    converted_model_str = C.convert_version(model_str, target_version)
IndexError: Input s is undefined!

• ONNX model shared-bn-arrays-v11.zip

Expected behavior

The script should not fail and properly upgrade the model (or at least fail saying there are no adapters)