yolov8 安卓端推理bug

[hwddean]

yolov8在pc端可以正常推理，但是在安卓端模型加载没有问题，检测没有框，换了好几个版本的库依然没有检测框

[nihui]

yolov8 example https://github.com/Tencent/ncnn/pull/5506

[nihui]

hi, yolov8 examples are updated with full support for detection, segmentation, classification, pose estimation and obb https://github.com/Tencent/ncnn/tree/master/examples

android demo https://github.com/nihui/ncnn-android-yolov8

detailed instruction (zh) https://zhuanlan.zhihu.com/p/16030630352