修复callback连续两次加入的bug

fastNLP/core/callback.py

@@ -324,7 +324,7 @@ class CallbackManager(Callback):
         self._env = env
         self.callbacks = []
         if callbacks:
-            self.callbacks += self.prepare_callbacks(callbacks)
+            self.prepare_callbacks(callbacks)
 
     def prepare_callbacks(self, callbacks):
         if not callbacks: