fix loss (#2614)

ultralytics/yolo/utils/loss.py

@@ -34,7 +34,7 @@ class BboxLoss(nn.Module):
 
     def forward(self, pred_dist, pred_bboxes, anchor_points, target_bboxes, target_scores, target_scores_sum, fg_mask):
         """IoU loss."""
-        weight = target_scores.sum(-1)[fg_mask]
+        weight = target_scores.sum(-1)[fg_mask].unsqueeze(-1)
         iou = bbox_iou(pred_bboxes[fg_mask], target_bboxes[fg_mask], xywh=False, CIoU=True)
         loss_iou = ((1.0 - iou) * weight).sum() / target_scores_sum