Add Optional to remaining types (#37808)

More Optional typing

Signed-off-by: cyy <cyyever@outlook.com>

examples/legacy/seq2seq/run_distributed_eval.py

@@ -19,6 +19,7 @@ import time
 from json import JSONDecodeError
 from logging import getLogger
 from pathlib import Path
+from typing import Optional
 
 import torch
 from torch.utils.data import DataLoader
@@ -54,7 +55,7 @@ def eval_data_dir(
     task="summarization",
     local_rank=None,
     num_return_sequences=1,
-    dataset_kwargs: dict = None,
+    dataset_kwargs: Optional[dict] = None,
     prefix="",
     **generate_kwargs,
 ) -> dict: