[V1] Implement Cascade Attention (#11635)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

tests/v1/e2e/test_cascade_attention.py

@@ -0,0 +1,22 @@
+from vllm import LLM, SamplingParams
+
+
+def test_cascade_attention(example_system_message, monkeypatch):
+    prompt = "\n<User>: Implement fibonacci sequence in Python.\n<Claude>:"
+
+    with monkeypatch.context() as m:
+        m.setenv("VLLM_USE_V1", "1")
+
+        llm = LLM(model="Qwen/Qwen2-1.5B-Instruct")
+        sampling_params = SamplingParams(temperature=0.0, max_tokens=100)
+
+        # No cascade attention.
+        single_prompt = [example_system_message + prompt]
+        responses = llm.generate(single_prompt, sampling_params)
+        ref_output = responses[0].outputs[0].text
+
+        # (Probably) Use cascade attention.
+        prompts = [example_system_message + prompt] * 64
+        responses = llm.generate(prompts, sampling_params)
+        for response in responses:
+            assert response.outputs[0].text == ref_output