update symlink option for save_last in ModelCheckpoint (#11319)

Signed-off-by: paul-gibbons <[email protected]>

nemo/lightning/pytorch/callbacks/model_checkpoint.py

@@ -17,7 +17,7 @@
 import shutil
 from datetime import timedelta
 from pathlib import Path
-from typing import Any, Dict, Iterable, List, Optional, Union
+from typing import Any, Dict, Iterable, List, Literal, Optional, Union
 
 import lightning.pytorch
 import torch
@@ -63,7 +63,7 @@ def __init__(
         self,
         monitor: Optional[str] = "val_loss",
         verbose: bool = True,
-        save_last: Optional[bool] = True,
+        save_last: Optional[Union[bool, Literal["link"]]] = True,
         save_top_k: int = 3,
         save_weights_only: bool = False,  ## TODO: check support
         mode: str = "min",