[Misc] enhance type hint for rearrange return value (#23519)

Signed-off-by: Andy Xie <andy.xning@gmail.com>

vllm/distributed/eplb/eplb_state.py

@@ -409,12 +409,14 @@ class EplbState:
             self.expert_rearrangement_step = 0
             self.rearrange(model)
 
-    def rearrange(self,
+    def rearrange(
+        self,
         model: MixtureOfExperts,
         is_profile: bool = False,
         execute_shuffle: bool = True,
         global_expert_load: Optional[torch.Tensor] = None,
-                  rank_mapping: Optional[dict[int, int]] = None) -> None:
+        rank_mapping: Optional[dict[int,
+                                    int]] = None) -> Optional[torch.Tensor]:
         """
         Rearrange the experts according to the current load.
         """
@@ -548,6 +550,7 @@ class EplbState:
                 " (profile) " if is_profile else " ",
                 time_end - time_start,
             )
+        return None
 
     @staticmethod
     def recv_state() -> tuple[torch.Tensor, torch.Tensor]: