ncnn-yolov8 detection and segmentation demo

The yolov8 object detection and segmentation

1.Android yolov8 detection demo
2.yolov8s-seg.cpp
3.yolov8s-obb.cpp

convert to onnx for ncnn

1.change c2f split to slice

2.for Detection model change class Detect output

3.for Segmentation model change class Detect output and Segment output

screenshot

Reference：

https://github.com/nihui/ncnn-android-nanodet
https://github.com/Tencent/ncnn
https://github.com/ultralytics/assets/releases/tag/v0.0.0
https://github.com/facebookresearch/detectron2