[RLHF] use worker_extension_cls for compatibility with V0 and V1 (#14185)

Signed-off-by: youkaichao <youkaichao@gmail.com>

vllm/config.py

@@ -1366,6 +1366,7 @@ class ParallelConfig:
     # will be determined based on the platform.
     worker_cls: str = "auto"
     sd_worker_cls: str = "auto"
+    worker_extension_cls: str = ""
 
     # world_size is TPxPP, it affects the number of workers we create.
     world_size: int = field(init=False)
@@ -1523,6 +1524,9 @@ class ParallelConfig:
             raise ValueError("Unable to use nsight profiling unless workers "
                              "run with Ray.")
 
+        assert isinstance(self.worker_extension_cls, str), (
+            "worker_extension_cls must be a string (qualified class name).")
+
 
 @dataclass
 class SchedulerConfig: