Standardize warnings with WARNING ⚠️ ... (#9467)

* Standardize warnings

* [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

@@ -186,7 +186,7 @@ class ConfusionMatrix:
         # fn = self.matrix.sum(0) - tp  # false negatives (missed detections)
         return tp[:-1], fp[:-1]  # remove background class
 
-    @TryExcept('WARNING: ConfusionMatrix plot failure: ')
+    @TryExcept('WARNING ⚠️ ConfusionMatrix plot failure: ')
     def plot(self, normalize=True, save_dir='', names=()):
         import seaborn as sn