[Frontend] Support setting logprobs to -1 (#25031)

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

tests/entrypoints/openai/test_chat_echo.py

@@ -99,3 +99,26 @@ async def test_prompt_logprobs(client: openai.AsyncOpenAI):
 
     assert completion.prompt_logprobs is not None
     assert len(completion.prompt_logprobs) > 0
+
+
+@pytest.mark.asyncio
+async def test_top_logprobs(client: openai.AsyncOpenAI):
+    messages = [{
+        "role": "system",
+        "content": "You are a helpful assistant."
+    }, {
+        "role": "user",
+        "content": "Beijing is the capital of which country?"
+    }]
+
+    completion = await client.chat.completions.create(
+        model=MODEL_NAME,
+        messages=messages,
+        extra_body={
+            "top_logprobs": -1,
+            "logprobs": "true",
+        },
+    )
+    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