[Bugfix]: During testing, use pytest monkeypatch for safely overriding the env var that indicates the vLLM backend (#5210)

tests/kernels/utils.py

@@ -0,0 +1,22 @@
+"""Kernel test utils"""
+
+import pytest
+
+STR_BACKEND_ENV_VAR: str = "VLLM_ATTENTION_BACKEND"
+STR_FLASH_ATTN_VAL: str = "FLASH_ATTN"
+STR_INVALID_VAL: str = "INVALID"
+
+
+def override_backend_env_variable(mpatch: pytest.MonkeyPatch,
+                                  backend_name: str) -> None:
+    '''
+    Override the environment variable indicating the vLLM backend temporarily,
+    using pytest monkeypatch to ensure that the env vars get
+    reset once the test context exits.
+
+    Arguments:
+
+    * mpatch: pytest monkeypatch instance
+    * backend_name: attention backend name to force
+    '''
+    mpatch.setenv(STR_BACKEND_ENV_VAR, backend_name)