Frontend: Adding LM Format Enforcer support to V1 engine (#22564)

Signed-off-by: Noam Gat <noamgat@gmail.com>
Co-authored-by: Russell Bryant <rbryant@redhat.com>
Co-authored-by: Cyrus Leung <tlleungac@connect.ust.hk>

vllm/v1/structured_output/__init__.py

@@ -108,6 +108,14 @@ class StructuredOutputManager:
                     tokenizer=self.tokenizer,
                     vocab_size=vocab_size,
                 )
+            elif backend == "lm-format-enforcer":
+                from vllm.v1.structured_output.backend_lm_format_enforcer import (  # noqa: E501
+                    LMFormatEnforcerBackend)
+                self.backend = LMFormatEnforcerBackend(
+                    self.vllm_config,
+                    tokenizer=self.tokenizer,
+                    vocab_size=vocab_size,
+                )
             else:
                 raise ValueError(
                     f"Unsupported structured output backend: {backend}")