Fix the prefix indices (#1037)

python/sglang/srt/mem_cache/radix_cache.py

@@ -22,7 +22,7 @@ The radix tree data structure for managing the KV cache.
 import heapq
 import time
 from collections import defaultdict
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Callable, List, Optional
 
 import torch