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

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

benchmarks/benchmark_batch_invariance.py

@@ -104,7 +104,6 @@ def run_benchmark_with_batch_invariant(
     random.seed(seed)
 
     # Set environment variables
-    os.environ["VLLM_ATTENTION_BACKEND"] = backend
     if batch_invariant:
         os.environ["VLLM_BATCH_INVARIANT"] = "1"
     else:
@@ -140,6 +139,7 @@ def run_benchmark_with_batch_invariant(
             max_model_len=max_model_len,
             dtype="bfloat16",
             tensor_parallel_size=tp_size,
+            attention_config={"backend": backend},
             enable_prefix_caching=False,
         )
         init_time = time.perf_counter() - start_init

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