fix: force synchronization between TP workers when update_weights (#6626)

Co-authored-by: dangkai.dk <dangkai.dk@alibaba-inc.com>

python/sglang/srt/managers/scheduler.py

@@ -2235,6 +2235,7 @@ class Scheduler(
                 assert flash_cache_success, "Cache flush failed after updating weights"
         else:
             logger.error(message)
+        barrier(group=self.tp_cpu_group)
         return UpdateWeightsFromTensorReqOutput(success, message)
 
     def get_weights_by_name(self, recv_req: GetWeightsByNameReqInput):