ONNX export for LazyConfigs

## 📚 Documentation Issue

I cannot find any information on exporting a custom model that was trained with LazyConfig system. The export_model.py only supports YACS-Configs. Am I missing something?

* Provide a link to an existing documentation/comment/tutorial:
https://detectron2.readthedocs.io/en/latest/tutorials/deployment.html
* How should the above documentation/comment/tutorial improve:
Add ONNX model export tutorial for LazyConfigs