修复TopkSaver在CheckpointCallbackk中的bug;修改Saver对象，使得其方便被直接使用

fastNLP/core/callbacks/checkpoint_callback.py

@@ -91,9 +91,10 @@ class CheckpointCallback(Callback):
         else:
             on_exceptions = []
 
-        self.topk_saver = TopkSaver(topk, monitor, larger_better, folder, only_state_dict,
-                                    model_save_fn, save_evaluate_results,
-                                    save_object, **kwargs)
+        self.topk_saver = TopkSaver(topk=topk, monitor=monitor, larger_better=larger_better, folder=folder,
+                                    save_object=save_object, only_state_dict=only_state_dict, model_save_fn=model_save_fn,
+                                    save_evaluate_results=save_evaluate_results, **kwargs)
+
         self.topk = topk
         self.save_object = save_object
 

fastNLP/core/callbacks/topk_saver.py

@@ -169,11 +169,11 @@ class TopkQueue:
 
 
 class TopkSaver(MonitorUtility, Saver):
-    def __init__(self, topk:int, monitor:str, larger_better:bool=True, folder:str=None, save_object:str='model',
+    def __init__(self, topk:int=0, monitor:str=None, larger_better:bool=True, folder:str=None, save_object:str='model',
                  only_state_dict:bool=True, model_save_fn:Callable=None, save_evaluate_results:bool=True,
                  **kwargs):
         """
-        用来保存识别 topk 模型并保存，也可以仅当一个保存 saver 使用。保存路径为
+        用来识别 topk 模型并保存，也可以仅当一个保存 Saver 使用。保存路径为
         - folder/
             - YYYY-mm-dd-HH_MM_SS_fffff/  # 自动根据当前脚本的启动时间创建的
                  - {save_object}-epoch_{epoch_idx}-batch_{global_batch_idx}-{topk_monitor}_{monitor_value}/  # 满足topk条件存储文件名