yolov5 knowledge distillation (offline)
usage: train with --t_weights xxx.pt and --kd_factor floatVal
compatible yolov5: https://github.com/ultralytics/yolov5/tree/f7bc685c2c0f57005b83355715cb7282e61416eb
reference: https://github.com/Syencil/mobile-yolov5-pruning-distillation