[Chore] Update more locations to use attention_config.backend (#31153)

Signed-off-by: DarkLight1337 <tlleungac@connect.ust.hk>

tests/compile/distributed/test_fusions_e2e.py

@@ -557,7 +557,8 @@ def test_rms_group_quant(
     # To capture subprocess logs, we need to know whether spawn or fork is used.
     # Force spawn as it is more general.
     monkeypatch.setenv("VLLM_WORKER_MULTIPROC_METHOD", "spawn")
-    monkeypatch.setenv("VLLM_ATTENTION_BACKEND", backend.name)
+
+    model_kwargs["attention_config"] = {"backend": backend.name}
 
     compilation_config = CompilationConfig(
         # Testing properties