add default value

train.py

@@ -730,6 +730,7 @@ def main():
             _logger.warning(
                 "You've requested to log metrics to wandb but package not found. "
                 "Metrics not being logged to wandb, try `pip install wandb`")
+    tensorboard_writer = None
     if should_log_to_tensorboard(args):
         if has_tensorboard:
             tensorboard_writer = SummaryWriter(args.log_tensorboard)