[V1] Delay all xgrammar usage until needed (#14616)

Signed-off-by: Russell Bryant <rbryant@redhat.com>

vllm/v1/structured_output/__init__.py

@@ -14,7 +14,6 @@ from vllm.v1.structured_output.grammar import Grammar, StructuredOutputOptions
 if TYPE_CHECKING:
     import numpy as np
     import numpy.typing as npt
-    import torch
     import xgrammar as xgr
 
     from vllm.v1.request import Request
@@ -27,14 +26,18 @@ logger = init_logger(__name__)
 class StructuredOutputManager:
 
     def __init__(self, vllm_config: VllmConfig):
-        tokenizer_group = init_tokenizer_from_configs(
-            model_config=vllm_config.model_config,
-            scheduler_config=vllm_config.scheduler_config,
-            parallel_config=vllm_config.parallel_config,
-            lora_config=vllm_config.lora_config)  # type: ignore[arg-type]
-        tokenizer_group.ping()
         self.vocab_size = vllm_config.model_config.get_vocab_size()
         self.vllm_config = vllm_config
+        self.init_complete = False
+
+    def _delayed_init(self):
+        """Initialization delayed until we know it is needed."""
+        tokenizer_group = init_tokenizer_from_configs(
+            model_config=self.vllm_config.model_config,
+            scheduler_config=self.vllm_config.scheduler_config,
+            parallel_config=self.vllm_config.parallel_config,
+            lora_config=self.vllm_config.lora_config)  # type: ignore[arg-type]
+        tokenizer_group.ping()
 
         tokenizer = tokenizer_group.get_lora_tokenizer(None)
         tokenizer_info = xgr.TokenizerInfo.from_huggingface(
@@ -47,12 +50,21 @@ class StructuredOutputManager:
         # compilation, so we set it to half the number of CPUs.
         max_workers = max(1, (multiprocessing.cpu_count() + 1) // 2)
         self.executor = ThreadPoolExecutor(max_workers=max_workers)
-        self._grammar_bitmask: Optional[torch.Tensor] = None
+        self._grammar_bitmask = xgr.allocate_token_bitmask(
+            self.vllm_config.scheduler_config.max_num_seqs, self.vocab_size)
+
+        self.init_complete = True
 
     def grammar_init(self, request: Request) -> None:
         if request.structured_output_request is None:
             return
 
+        # The first time this is called, we need to finish initialization
+        # of xgrammar. We defer it to avoid the import of xgrammar and
+        # initialization cost if it is not going to be used.
+        if not self.init_complete:
+            self._delayed_init()
+
         grammar: Future[Grammar] = self.executor.submit(
             self._async_create_grammar, request)
         request.structured_output_request.grammar = grammar  # type: ignore[assignment]
@@ -100,11 +112,6 @@ class StructuredOutputManager:
         if not structured_output_request_ids:
             return None
 
-        if self._grammar_bitmask is None:
-            self._grammar_bitmask = xgr.allocate_token_bitmask(
-                self.vllm_config.scheduler_config.max_num_seqs,
-                self.vocab_size)
-
         # Fill the bitmask using the index of each request equal to its
         # position in the batch. Resize the bitmask down to the size of
         # the batch.