[Frontend][Core] Move guided decoding params into sampling params (#8252)

Signed-off-by: Joe Runde <Joseph.Runde@ibm.com>
Co-authored-by: Nick Hill <nickhill@us.ibm.com>

vllm/engine/async_llm_engine.py

@@ -20,6 +20,8 @@ from vllm.executor.ray_utils import initialize_ray_cluster
 from vllm.inputs import PromptType
 from vllm.logger import init_logger
 from vllm.lora.request import LoRARequest
+from vllm.model_executor.guided_decoding import (
+    get_guided_decoding_logits_processor)
 from vllm.model_executor.layers.sampler import SamplerOutput
 from vllm.outputs import EmbeddingRequestOutput, RequestOutput
 from vllm.pooling_params import PoolingParams
@@ -477,6 +479,18 @@ class _AsyncLLMEngine(LLMEngine):
         )
         processed_inputs = self.input_processor(preprocessed_inputs)
 
+        if isinstance(params, SamplingParams) and \
+            params.guided_decoding is not None:
+            # Guided decoding has an async implementation for building logits
+            # processors in a separate threadpool.
+            # We want to invoke that here instead of using the blocking
+            # implementation in the LLMEngine
+            params = await build_guided_decoding_logits_processor_async(
+                sampling_params=params,
+                tokenizer=self.get_tokenizer(lora_request),
+                default_guided_backend=self.decoding_config.
+                guided_decoding_backend)
+
         self._add_processed_request(
             request_id=request_id,
             processed_inputs=processed_inputs,
@@ -494,6 +508,36 @@ class _AsyncLLMEngine(LLMEngine):
         self.model_executor.check_health()
 
 
+async def build_guided_decoding_logits_processor_async(
+        sampling_params: SamplingParams, tokenizer: AnyTokenizer,
+        default_guided_backend: str) -> SamplingParams:
+    """Constructs logits processors based on the guided_decoding,
+    logits_bias, and allowed_token_ids fields in sampling_params. Deletes
+    those fields and adds the constructed logits processors to the
+    logits_processors field. Modifies sampling params in-place and returns
+    the modified sampling params."""
+    if (guided_decoding := sampling_params.guided_decoding) is None:
+        return sampling_params
+
+    logger.debug("Building guided decoding logits processor. "
+                 "Params: %s", guided_decoding)
+
+    guided_decoding.backend = guided_decoding.backend or default_guided_backend
+
+    processor = await get_guided_decoding_logits_processor(
+        guided_params=guided_decoding, tokenizer=tokenizer)
+
+    if processor:
+        if sampling_params.logits_processors is None:
+            sampling_params.logits_processors = []
+        sampling_params.logits_processors.append(processor)
+
+    # Unset guided decoding params after constructing the lp from them
+    sampling_params.guided_decoding = None
+
+    return sampling_params
+
+
 class AsyncLLMEngine:
     """An asynchronous wrapper for :class:`LLMEngine`.