Refactor for simplification (#9054)

* Refactor for simplification

* cleanup

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

utils/metrics.py

@@ -141,7 +141,7 @@ class ConfusionMatrix:
         """
         if detections is None:
             gt_classes = labels.int()
-            for i, gc in enumerate(gt_classes):
+            for gc in gt_classes:
                 self.matrix[self.nc, gc] += 1  # background FN
             return