Add pydantic support in response_format

[lhoestq]

• support passing a Pydantic schema in response_format in InferenceClient.chat_completion

class MyExampleResponseFormat(BaseModel):
    foo: str
    bar: str

response = client.chat_completion(messages=messages, response_format=MyExampleResponseFormat)

• added response.choices[0].message.parsed (and response.choices[0].message.refusal if the generation failed as in the openai client)

message = response.choices[0].message
if message.parsed:
    print(message.parsed)
else:
    print(message.refusal)

close #2646

[HuggingFaceDocBuilderDev]

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[Wauplin]

As discussed, we'll have to see what we can do for this. Especially because ChatCompletionOutputMessage is an auto-generated class (also the stream version ChatCompletionStreamOutputMessage would have to be updated). And do the same for InferenceClient.text_generation(..., grammar=...).

[lhoestq]

Ok, note that this is mostly useful for the non-streaming case imo