How to export coreml model

Thank you for your contribution. How did you convert the YOLOv7 model to a CoreML model?

I followed the [official YOLOv7](https://github.com/WongKinYiu/yolov7) documentation, but the CoreML model I generated couldn't be directly substituted for use.

```
python export.py --weights path/to/best.pt --include-nms
```