YOLOv8 model metadata on export

[Burhan-Q]

From a discussion in the Ultralytics Discord, a user was having issues with a model exported after sparseml training. I cooked up a quick process to add the metadata to the output ONNX model and thought it would be worthwhile to add here.

Of course feel free to make changes as needed to comply with code standards/designs, I tried to minimize as much as possible, but suspect there could be other preferences. Let me know if there are any questions or changes needed 🚀