[Core] Cache `vllm_is_batch_invariant` (#28304)

Signed-off-by: Lukas Geiger <lukas.geiger94@gmail.com>

vllm/model_executor/layers/batch_invariant.py

@@ -4,6 +4,7 @@ import contextlib
 import os
 from collections import namedtuple
 from collections.abc import Callable
+from functools import cache
 from typing import Any
 
 import torch
@@ -857,6 +858,7 @@ def get_batch_invariant_attention_block_size() -> AttentionBlockSize:
     return AttentionBlockSize(block_m=16, block_n=16)
 
 
+@cache
 def vllm_is_batch_invariant():
     env_key = "VLLM_BATCH_INVARIANT"
     is_overridden = False