onnxruntime

Fix opset version of the optimizer in function generate_artifacts (#18300)

### Description
`generate_artifacts` generates 4 graphs for training. All graphs should
share the same opset version, the one coming from the model to train,
but the optimizer is left undefined. onnxruntime is using the latest
version defined by onnx but onnxruntime does not necessarily support it.

### Motivation and Context
The code does not let the user change it.