[Frontend] add json_schema support from OpenAI protocol

[rockwotj]

OpenAI now supports json_schema and since vLLM already supports this
functionality add the ability to plumb the new OpenAI protocol field to
vLLM's functionality.

FIX #7656

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[github-actions]

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[simon-mo]

thank you for opening this PR! lmk when it is ready for review and test passed!

[rockwotj]

/ready

[rockwotj]

Verified this works locally:

$ curl http://localhost:8000/v1/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "HuggingFaceH4/zephyr-7b-beta",
        "prompt": "what is 1+1? please respond in JSON ONLY, the format is {\"result\": 2}",
        "max_tokens": 64,
        "temperature": 0,
        "response_format": {
          "type": "json_schema",
          "json_schema": {"name":"foo_test", "schema":{"type":"object", "properties":{"result":{"type":"integer"}}}}
        }
    }'
{"id":"cmpl-f727dcae29bd43ff9b63979e69f6c6a9","object":"text_completion","created":1724163952,"model":"HuggingFaceH4/zephyr-7b-beta","choices":[{"index":0,"tex
t":"{\"result\": 2}","logprobs":null,"finish_reason":"stop","stop_reason":null,"prompt_logprobs":null}],"usage":{"prompt_tokens":24,"total_tokens":31,"completi
on_tokens":7}}

$ curl http://localhost:8000/v1/completions \
    -H "Content-Type: application/json" \
    -d '{
        "model": "HuggingFaceH4/zephyr-7b-beta",
        "prompt": "what is 1+1? please respond in JSON ONLY, the format is {\"result\": 2}",
        "max_tokens": 64,
        "temperature": 0,
        "response_format": {
          "type": "json_schema",
          "json_schema": {"name":"foo_test", "schema":{"type":"object", "properties":{"resul":{"type":"integer"}}}}
        }
    }'
{"id":"cmpl-0ba2f630ad664ffb8fe6cac44e7b8aca","object":"text_completion","created":1724163965,"model":"HuggingFaceH4/zephyr-7b-beta","choices":[{"index":0,"text":"{\"resul\": 2}","logprobs":null,"finish_reason":"stop","stop_reason":null,"prompt_logprobs":null}],"usage":{"prompt_tokens":24,"total_tokens":33,"completion_tokens":9}}

[rockwotj]

This is ready @simon-mo

[Quang-elec44]

Hi! Does this actually work? When I ran the below script, the model generated the redundant ```json token.

import openai

from pydantic import BaseModel

client = openai.OpenAI(
    base_url="http://localhost:8008/v1",
    api_key="sk-no-key-required"
)

class Players(BaseModel):
    names: list[str]
    

completion = client.chat.completions.create(
    model="gpt-4o",
    messages=[
        {"role": "user", "content": "Give me three football players' name and return the result as a JSON object with the key is `names`"}
    ],
    temperature=0.0,
    max_tokens=256,
    extra_body={
        "response_format": {
            "type": "json_schema",
            "json_schema": {
                "name": Players.__name__,
                "schema": Players.model_json_schema()
        }
    }
    }
)
print(completion.choices[0].message.content)

Output:

'```json\n{\n  "names": ["Lionel Messi", "Cristiano Ronaldo", "Neymar"]\n}\n```'

BTW, should it be

extra_body={
        "response_format": {
            "type": "json_schema",
            "json_schema": {
                "name": Players.__name__,
                "schema": Players.model_json_schema()
        }

or

response_format={
      "type": "json_schema",
      "json_schema": {
          "name": Players.__name__,
          "schema": Players.model_json_schema()
  }

None of the above arguments work as expected. The document doesn't provide a clear instruction on how to use this feature, so can you guys update this? Thanks in advance

The model is: Qwen/Qwen2.5-32B-Instruct-AWQ and the vllm version is 0.6.3.post1

@simon-mo