[V1][Structured Output] Clear xgrammar compiler object when engine core shut...

[V1][Structured Output] Clear xgrammar compiler object when engine core shut down to avoid nanobind leaked warning (#16954)
Signed-off-by: shen-shanshan <467638484@qq.com>

vllm/v1/engine/core.py

@@ -253,6 +253,7 @@ class EngineCore:
         return engine_core_outputs
 
     def shutdown(self):
+        self.structured_output_manager.clear_backend()
         if self.model_executor:
             self.model_executor.shutdown()
 

vllm/v1/structured_output/__init__.py

@@ -107,3 +107,7 @@ class StructuredOutputManager:
         # np.ndarray, because that is much more efficient for serialization
         # and deserialization when sending this to the GPU workers.
         return bitmask_tensor.numpy()
+
+    def clear_backend(self) -> None:
+        if self.backend is not None:
+            self.backend.destroy()

vllm/v1/structured_output/backend_guidance.py

@@ -108,6 +108,9 @@ class GuidanceBackend(StructuredOutputBackend):
         return llguidance_torch.allocate_token_bitmask(
             max_num_seqs, self.ll_tokenizer.vocab_size)
 
+    def destroy(self):
+        pass
+
 
 @dataclass
 class GuidanceGrammar(StructuredOutputGrammar):

vllm/v1/structured_output/backend_types.py

@@ -87,3 +87,9 @@ class StructuredOutputBackend(ABC):
             max_num_seqs (int): The maximum number of sequences for which
               to allocate the bitmask.
         """
+
+    @abstractmethod
+    def destroy(self):
+        """
+        Backend-specific cleanup.
+        """

vllm/v1/structured_output/backend_xgrammar.py

@@ -124,6 +124,9 @@ class XgrammarBackend(StructuredOutputBackend):
     def allocate_token_bitmask(self, max_num_seqs: int):
         return xgr.allocate_token_bitmask(max_num_seqs, self.vocab_size)
 
+    def destroy(self):
+        del self.compiler
+
 
 @dataclass
 class XgrammarGrammar(StructuredOutputGrammar):