[Kernel] Use flashinfer for decoding (#4353)

Co-authored-by: LiuXiaoxuanPKU <llilyliupku@gmail.com>

tests/basic_correctness/test_basic_correctness.py

@@ -2,12 +2,15 @@
 
 Run `pytest tests/basic_correctness/test_basic_correctness.py`.
 """
+import os
+
 import pytest
 
 MODELS = [
     "facebook/opt-125m",
     "meta-llama/Llama-2-7b-hf",
 ]
+VLLM_ATTENTION_BACKEND = "VLLM_ATTENTION_BACKEND"
 
 
 @pytest.mark.parametrize("model", MODELS)
@@ -23,11 +26,18 @@ def test_models(
     max_tokens: int,
     enforce_eager: bool,
 ) -> None:
+    backend_by_env_var = os.getenv(VLLM_ATTENTION_BACKEND)
+    if backend_by_env_var == "FLASHINFER" and enforce_eager is False:
+        pytest.skip("Skipping non-eager test for FlashInferBackend.")
+
     hf_model = hf_runner(model, dtype=dtype)
     hf_outputs = hf_model.generate_greedy(example_prompts, max_tokens)
     del hf_model
 
-    vllm_model = vllm_runner(model, dtype=dtype, enforce_eager=enforce_eager)
+    vllm_model = vllm_runner(model,
+                             dtype=dtype,
+                             enforce_eager=enforce_eager,
+                             gpu_memory_utilization=0.7)
     vllm_outputs = vllm_model.generate_greedy(example_prompts, max_tokens)
     del vllm_model