[train] fix StartTracebackWithWorkerRank serialization (#48548)

Signed-off-by: Matthew Deng <[email protected]>

python/ray/air/_internal/util.py

@@ -40,6 +40,9 @@ def __init__(self, worker_rank: int) -> None:
         super().__init__()
         self.worker_rank = worker_rank
 
+    def __reduce__(self):
+        return (self.__class__, (self.worker_rank,))
+
 
 def skip_exceptions(exc: Optional[Exception]) -> Exception:
     """Skip all contained `StartTracebacks` to reduce traceback output.