Maintaining versioning for ONNX models

[vinitra-zz]

In the model zoo, a contributor normally submits a model and also maintains opset versions of each model.

Because of difference in contribution levels between contributors, some models have a lot more models at different versions than others.

1. Can we take advantage of the version converters to automate this process?
2. What role should the Model Zoo SIG play in the maintenance of these models for new ONNX / ONNX Runtime releases?