[core] allow callable in collective_rpc (#12151)

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

tests/engine/test_custom_executor.py

@@ -1,6 +1,6 @@
 import asyncio
 import os
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Any, Callable, Dict, List, Optional, Tuple, Union
 
 import pytest
 
@@ -18,7 +18,7 @@ class Mock:
 class CustomUniExecutor(UniProcExecutor):
 
     def collective_rpc(self,
-                       method: str,
+                       method: Union[str, Callable],
                        timeout: Optional[float] = None,
                        args: Tuple = (),
                        kwargs: Optional[Dict] = None) -> List[Any]: