Fix yolov8.cpp larger object boundingbox error

[caoshihao996]

Yolov8 has three different heads and output different size feature maps. So the part of boundingboxs needs to be adjusted accordingly.

[tencent-adm]

All committers have signed the CLA.

[whyb]

@caoshihao996 @nihui 并没有从yolov8的ultralytics官方ort推理示例中寻找到类似您pr中所提供的根据num_anchors（或称strideNum）的值针对boundingboxs进行坐标和宽高的调整之代码，只有针对fp16和fp32的处理，如果有更多其他可供参考的文档或者Torch Yolov8后处理的代码，欢迎提出来共同学习讨论~

yolov8的ultralytics官方ort推理示例源码： https://github.com/ultralytics/ultralytics/blob/main/examples/YOLOv8-ONNXRuntime-CPP/inference.cpp#L263

[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