[examples/seq2seq] fix PL deprecation warning (#8577)

* fix deprecation warning

* fix

examples/seq2seq/callbacks.py

 import logging
-import os
 from pathlib import Path
 
 import numpy as np
@@ -98,7 +97,8 @@ def get_checkpoint_callback(output_dir, metric, save_top_k=1, lower_is_better=Fa
         )
 
     checkpoint_callback = ModelCheckpoint(
-        filepath=os.path.join(output_dir, exp),
+        dirpath=output_dir,
+        filename=exp,
         monitor=f"val_{metric}",
         mode="min" if "loss" in metric else "max",
         save_top_k=save_top_k,