Fix 6 Flake8 issues (#6541)

* F541

* F821

* F841

* E741

* E302

* E722

* Apply suggestions from code review

* Update general.py

* Update datasets.py

* Update export.py

* Update plots.py

* Update plots.py

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

utils/metrics.py

@@ -239,6 +239,7 @@ def bbox_iou(box1, box2, x1y1x2y2=True, GIoU=False, DIoU=False, CIoU=False, eps=
         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
     """