[Misc] centralize all usage of environment variables (#4548)

vllm/attention/selector.py

@@ -1,18 +1,16 @@
 import enum
-import os
 from functools import lru_cache
 from typing import Type
 
 import torch
 
+import vllm.envs as envs
 from vllm.attention.backends.abstract import AttentionBackend
 from vllm.logger import init_logger
 from vllm.utils import is_cpu, is_hip
 
 logger = init_logger(__name__)
 
-VLLM_ATTENTION_BACKEND = "VLLM_ATTENTION_BACKEND"
-
 
 class _Backend(enum.Enum):
     FLASH_ATTN = enum.auto()
@@ -79,7 +77,7 @@ def _which_attn_to_use(dtype: torch.dtype) -> _Backend:
             "package is not found. Please install it for better performance.")
         return _Backend.XFORMERS
 
-    backend_by_env_var = os.getenv(VLLM_ATTENTION_BACKEND)
+    backend_by_env_var = envs.VLLM_ATTENTION_BACKEND
     if backend_by_env_var is not None:
         return _Backend[backend_by_env_var]