Set tb_writer to None in TensorBoardCallback.on_train_end() (#12963)

src/transformers/integrations.py

@@ -401,6 +401,7 @@ class TensorBoardCallback(TrainerCallback):
     def on_train_end(self, args, state, control, **kwargs):
         if self.tb_writer:
             self.tb_writer.close()
+            self.tb_writer = None
 
 
 class WandbCallback(TrainerCallback):