Fix TensorBoard graph UserWarning catch (#4513)

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

ultralytics/engine/trainer.py

@@ -10,6 +10,7 @@ import math
 import os
 import subprocess
 import time
+import warnings
 from copy import deepcopy
 from datetime import datetime, timedelta
 from pathlib import Path
@@ -378,7 +379,9 @@ class BaseTrainer:
 
             self.lr = {f'lr/pg{ir}': x['lr'] for ir, x in enumerate(self.optimizer.param_groups)}  # for loggers
 
-            self.scheduler.step()
+            with warnings.catch_warnings():
+                warnings.simplefilter('ignore')  # suppress 'Detected lr_scheduler.step() before optimizer.step()'
+                self.scheduler.step()
             self.run_callbacks('on_train_epoch_end')
 
             if RANK in (-1, 0):