[Misc] Convert use_structured_output property into constant (#23060)

Signed-off-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>

Author: WoosukKwon

vllm/v1/request.py

@@ -54,21 +54,23 @@ def __init__(
  time.time()
 
  self.status = RequestStatus.WAITING
- if sampling_params and sampling_params.guided_decoding is not None:
- self.status = RequestStatus.WAITING_FOR_FSM
+ self.use_structured_output = False
  self.events: list[EngineCoreEvent] = []
  self.stop_reason: Union[int, str, None] = None
 
  # P/D: Connector-specific KV transfer parameters.
  self.kv_transfer_params: Optional[dict[str, Any]] = None
 
  if pooling_params is not None:
+ # Pooling models.
  self.max_tokens = 1
  elif sampling_params is not None:
+ # Generative models.
  assert sampling_params.max_tokens is not None
  self.max_tokens = sampling_params.max_tokens
  if sampling_params.guided_decoding is not None:
  self.status = RequestStatus.WAITING_FOR_FSM
+ self.use_structured_output = True
 
  if sampling_params.extra_args is not None:
  self.kv_transfer_params = \
@@ -192,11 +194,6 @@ def get_num_encoder_tokens(self, input_id: int) -> int:
  num_tokens = self.mm_positions[input_id].length
  return num_tokens
 
- @property
- def use_structured_output(self) -> bool:
- return self.sampling_params is not None and \
- self.sampling_params.guided_decoding is not None
-
  def record_event(
  self,
  event_type: EngineCoreEventType,