[V1][Spec Decode] Ngram Spec Decode (#12193)

Signed-off-by: LiuXiaoxuanPKU <lilyliupku@gmail.com>

tests/v1/e2e/test_ngram_spec_decode.py

@@ -0,0 +1,49 @@
+# SPDX-License-Identifier: Apache-2.0
+import pytest
+
+from vllm import LLM, SamplingParams
+
+
+@pytest.fixture
+def test_prompts():
+    return [
+        "Can you repeat the sentence ten times, this is a sentence.",
+        "Can you repeat the sentence ten times, this is a test.",
+    ]
+
+
+@pytest.fixture
+def sampling_config():
+    # Only support greedy for now
+    return SamplingParams(temperature=0, max_tokens=30, ignore_eos=False)
+
+
+@pytest.fixture
+def model_name():
+    return "meta-llama/Meta-Llama-3-8B-Instruct"
+
+
+def test_ngram_correctness(monkeypatch, test_prompts, sampling_config,
+                           model_name):
+    '''
+    Compare the outputs of a original LLM and a speculative LLM
+    should be the same when using ngram speculative decoding.
+    '''
+    with monkeypatch.context() as m:
+        m.setenv("VLLM_USE_V1", "1")
+
+        ref_llm = LLM(model=model_name)
+        ref_outputs = ref_llm.generate(test_prompts, sampling_config)
+        del ref_llm
+
+        spec_llm = LLM(model=model_name,
+                       speculative_model='[ngram]',
+                       ngram_prompt_lookup_max=5,
+                       ngram_prompt_lookup_min=3,
+                       num_speculative_tokens=3)
+        spec_outputs = spec_llm.generate(test_prompts, sampling_config)
+        for ref_output, spec_output in zip(ref_outputs, spec_outputs):
+            assert ref_output.outputs[0].text == spec_output.outputs[0].text, \
+                (f"ref_output: {ref_output.outputs[0].text},"
+                 f"spec_output: {spec_output.outputs[0].text}")
+        del spec_llm