[Misc] Throw error on unintended access to scheduler_config.max_model_len (#29771)

Signed-off-by: Wei Wei <wwei6@meta.com>

vllm/config/scheduler.py

@@ -274,3 +274,8 @@ class SchedulerConfig:
             )
 
         return self
+
+    def __getattribute__(self, name: str) -> Any:
+        if name == "max_model_len" or name == "is_encoder_decoder":
+            raise AttributeError(f"{name} is an init-only parameter. ")
+        return object.__getattribute__(self, name)