fix bug in mu_map trianing lib

mu_map/training/lib.py

@@ -71,7 +71,7 @@ class AbstractTraining:
         snapshot_dir: str,
         snapshot_epoch: int,
         early_stopping: Optional[int] = None,
-        logger: Optional[Logger],
+        logger: Optional[Logger] = None,
     ):
         self.epochs = epochs
         self.batch_size = batch_size