[Core][VLM] Test registration for OOT multimodal models (#8717)

Co-authored-by: DarkLight1337 <tlleungac@connect.ust.hk>

vllm/engine/arg_utils.py

@@ -183,6 +183,8 @@ class EngineArgs:
     def __post_init__(self):
         if self.tokenizer is None:
             self.tokenizer = self.model
+        from vllm.plugins import load_general_plugins
+        load_general_plugins()
 
     @staticmethod
     def add_cli_args(parser: FlexibleArgumentParser) -> FlexibleArgumentParser: