Update integrations.py (#9652)

File "/share/apps/anaconda3/envs/my_env/lib/python3.7/site-packages/transformers/integrations.py", line 419, in __init__
    self._SummaryWriter = SummaryWriter
UnboundLocalError: local variable 'SummaryWriter' referenced before assignment

src/transformers/integrations.py

@@ -415,8 +415,9 @@ class TensorBoardCallback(TrainerCallback):
                     self._SummaryWriter = SummaryWriter
                 except ImportError:
                     self._SummaryWriter = None
+        else:
+            self._SummaryWriter = None
         self.tb_writer = tb_writer
-        self._SummaryWriter = SummaryWriter
 
     def _init_summary_writer(self, args, log_dir=None):
         log_dir = log_dir or args.logging_dir