[Misc] Include matched stop string/token in responses (#2976)

Co-authored-by: Sahil Suneja <sahilsuneja@gmail.com>

tests/samplers/test_stop_reason.py

@@ -0,0 +1,59 @@
+"""Test the different finish_reason="stop" situations during generation:
+    1. One of the provided stop strings
+    2. One of the provided stop tokens
+    3. The EOS token
+
+Run `pytest tests/samplers/test_stop_reason.py`.
+"""
+
+import pytest
+import transformers
+
+from vllm import SamplingParams
+
+MODEL = "facebook/opt-350m"
+STOP_STR = "."
+SEED = 42
+MAX_TOKENS = 1024
+
+
+@pytest.fixture
+def vllm_model(vllm_runner):
+    vllm_model = vllm_runner(MODEL)
+    yield vllm_model
+    del vllm_model
+
+
+def test_stop_reason(vllm_model, example_prompts):
+    tokenizer = transformers.AutoTokenizer.from_pretrained(MODEL)
+    stop_token_id = tokenizer.convert_tokens_to_ids(STOP_STR)
+    llm = vllm_model.model
+
+    # test stop token
+    outputs = llm.generate(example_prompts,
+                           sampling_params=SamplingParams(
+                               seed=SEED,
+                               max_tokens=MAX_TOKENS,
+                               stop_token_ids=[stop_token_id]))
+    for output in outputs:
+        output = output.outputs[0]
+        assert output.finish_reason == "stop"
+        assert output.stop_reason == stop_token_id
+
+    # test stop string
+    outputs = llm.generate(example_prompts,
+                           sampling_params=SamplingParams(
+                               seed=SEED, max_tokens=MAX_TOKENS, stop="."))
+    for output in outputs:
+        output = output.outputs[0]
+        assert output.finish_reason == "stop"
+        assert output.stop_reason == STOP_STR
+
+    # test EOS token
+    outputs = llm.generate(example_prompts,
+                           sampling_params=SamplingParams(
+                               seed=SEED, max_tokens=MAX_TOKENS))
+    for output in outputs:
+        output = output.outputs[0]
+        assert output.finish_reason == "length" or (
+            output.finish_reason == "stop" and output.stop_reason is None)