Fix bug ultralytics#541 ultralytics#542 (ultralytics#545)

* fix ultralytics#541 ultralytics#542

* Update train.py

Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

Author: AlexWang1900

train.py

@@ -377,7 +377,7 @@ def train(hyp, tb_writer, opt, device):
 
                 # Save last, best and delete
                 torch.save(ckpt, last)
-                if (best_fitness == fi) and not final_epoch:
+                if best_fitness == fi: 
                     torch.save(ckpt, best)
                 del ckpt
         # end epoch ----------------------------------------------------------------------------------------------------

utils/utils.py

@@ -346,8 +346,8 @@ def bbox_iou(box1, box2, x1y1x2y2=True, GIoU=False, DIoU=False, CIoU=False):
             elif CIoU:  # https://github.com/Zzh-tju/DIoU-SSD-pytorch/blob/master/utils/box/box_utils.py#L47
                 v = (4 / math.pi ** 2) * torch.pow(torch.atan(w2 / h2) - torch.atan(w1 / h1), 2)
                 with torch.no_grad():
-                    alpha = v / (1 - iou + v)
-                return iou - (rho2 / c2 + v * alpha)  # CIoU
+                    alpha = v / (1 - iou + v + 1e-16)
+                return iou - (rho2 / c2 + v * alpha )  # CIoU
 
     return iou