Refactor/reduce G/C/D/IoU if: else statements (ultralytics#6087)

* Refactor the code to reduece else

* Update metrics.py

* Cleanup

Co-authored-by: Cmos <gen.chen@ubisoft.com>
Co-authored-by: Glenn Jocher <glenn.jocher@ultralytics.com>

utils/metrics.py

@@ -234,14 +234,10 @@ def bbox_iou(box1, box2, x1y1x2y2=True, GIoU=False, DIoU=False, CIoU=False, eps=
                 with torch.no_grad():
                     alpha = v / (v - iou + (1 + eps))
                 return iou - (rho2 / c2 + v * alpha)  # CIoU
-            else:
-                return iou - rho2 / c2  # DIoU
-        else:  # GIoU https://arxiv.org/pdf/1902.09630.pdf
-            c_area = cw * ch + eps  # convex area
-            return iou - (c_area - union) / c_area  # GIoU
-    else:
-        return iou  # IoU
-
+            return iou - rho2 / c2  # DIoU
+        c_area = cw * ch + eps  # convex area
+        return iou - (c_area - union) / c_area  # GIoU https://arxiv.org/pdf/1902.09630.pdf
+    return iou  # IoU
 
 def box_iou(box1, box2):
     # https://github.com/pytorch/vision/blob/master/torchvision/ops/boxes.py