put the shuffer into iter

ppocr/data/det/dataset_traversal.py

@@ -41,8 +41,8 @@ class TrainReader(object):
             label_infor_list = fin.readlines()        
         img_num = len(label_infor_list)
         img_id_list = list(range(img_num))        
-        random.shuffle(img_id_list)
         if sys.platform == "win32":
+            random.shuffle(img_id_list)
             print("multiprocess is not fully compatible with Windows."
                   "num_workers will be 1.")
             self.num_workers = 1