Just wanted to put this on the radar for the project - the developers of Complete IoU have extended it with clustered NMS:
"...we propose Cluster-NMS, where NMS during inference is done by implicitly clustering detected boxes and usually requires less iterations. Cluster-NMS is very efficient due to its pure GPU implementation, , and geometric factors can be incorporated to improve both AP and AR. In the experiments, CIoU loss and Cluster-NMS have been applied to state-of-the-art instance segmentation (e.g., YOLACT), and object detection (e.g., YOLO v3, SSD and Faster R-CNN) models. Taking YOLACT on MS COCO as an example, our method achieves performance gains as 1.7 AP and 6.2 AR100 for object detection..."

https://arxiv.org/abs/2005.03572
code:
https://github.com/Zzh-tju/CIoU