[CI] Remove unused imports with Ruff to pre-commit config, only to benchmarks/docs/examples folder (#3969)

benchmark/hicache/bench_serving.py

@@ -23,7 +23,7 @@ import warnings
 from argparse import ArgumentParser
 from dataclasses import dataclass, field
 from datetime import datetime
-from typing import Any, AsyncGenerator, Dict, List, Optional, Tuple, Union
+from typing import Any, AsyncGenerator, Dict, List, Optional, Tuple
 
 import aiohttp
 import numpy as np