[Bugfix] fixed top_logprobs: -1 does not appear to work as intended (#26470)

Signed-off-by: chaunceyjiang <chaunceyjiang@gmail.com>

tests/entrypoints/openai/test_chat_echo.py

@@ -7,12 +7,23 @@ import openai  # use the official client for correctness check
 import pytest
 import pytest_asyncio
 
+from vllm.config import ModelConfig
+
 from ...utils import RemoteOpenAIServer
 
 # # any model with a chat template should work here
 MODEL_NAME = "Qwen/Qwen2-1.5B-Instruct"
 
 
+def get_vocab_size(model_name):
+    config = ModelConfig(
+        model=model_name,
+        seed=0,
+        dtype="float16",
+    )
+    return config.get_vocab_size()
+
+
 @pytest.fixture(scope="module")
 def server():
     args = [
@@ -107,6 +118,7 @@ async def test_top_logprobs(client: openai.AsyncOpenAI):
     completion = await client.chat.completions.create(
         model=MODEL_NAME,
         messages=messages,
+        max_tokens=1,
         extra_body={
             "top_logprobs": -1,
             "logprobs": "true",
@@ -115,3 +127,6 @@ async def test_top_logprobs(client: openai.AsyncOpenAI):
     assert completion.choices[0].logprobs is not None
     assert completion.choices[0].logprobs.content is not None
     assert len(completion.choices[0].logprobs.content) > 0
+    assert len(
+        completion.choices[0].logprobs.content[0].top_logprobs
+    ) == get_vocab_size(MODEL_NAME)

vllm/entrypoints/openai/serving_chat.py

@@ -1643,7 +1643,7 @@ class OpenAIServingChat(OpenAIServing):
                 bytes=list(token.encode("utf-8", errors="replace")),
             )
             for i, p in enumerate(logprobs.items())
-            if top_logprobs and i < top_logprobs
+            if (top_logprobs and i < top_logprobs or top_logprobs == -1)
         ]
 
     def _create_chat_logprobs(