bugfix change annotation of ModelCheckpoint's save_last parameter, so…

… jsonargparse can correctly parse inputs such as true, false, etc...

before this change jsonargparse gave errors complaining that strings like 'true' or 'false' were given when the annotation expected Optional[Literal[True, False, 'link']]; this is in contrast to most other bool parameters in the ModelCheckpoint callback's constructor. I also added a test in tests/test_pytorch/checkpointing/test_model_checkpoint.py::test_save_last_cli

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -27,7 +27,7 @@
 from copy import deepcopy
 from datetime import timedelta
 from pathlib import Path
-from typing import Any, Dict, Literal, Optional, Set
+from typing import Any, Dict, Literal, Optional, Set, Union
 from weakref import proxy
 
 import torch
@@ -216,7 +216,7 @@ def __init__(
         filename: Optional[str] = None,
         monitor: Optional[str] = None,
         verbose: bool = False,
-        save_last: Optional[Literal[True, False, "link"]] = None,
+        save_last: Optional[Union[bool, Literal["link"]]] = None,
         save_top_k: int = 1,
         save_weights_only: bool = False,
         mode: str = "min",

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -18,6 +18,7 @@
 import time
 from argparse import Namespace
 from datetime import timedelta
+from inspect import signature
 from pathlib import Path
 from typing import Union
 from unittest import mock
@@ -28,6 +29,7 @@
 import pytest
 import torch
 import yaml
+from jsonargparse import ArgumentParser
 from lightning.fabric.utilities.cloud_io import _load as pl_load
 from lightning.pytorch import Trainer, seed_everything
 from lightning.pytorch.callbacks import ModelCheckpoint
@@ -43,6 +45,26 @@
     from omegaconf import Container, OmegaConf
 
 
+@pytest.mark.parametrize(
+    ("val", "expected"),
+    [
+        ("yes", True),
+        ("True", True),
+        ("true", True),
+        ("no", False),
+        ("false", False),
+        ("False", False),
+        ("link", "link"),
+    ],
+)
+def test_save_last_cli(val, expected):
+    annot = signature(ModelCheckpoint).parameters["save_last"].annotation
+    parser = ArgumentParser()
+    parser.add_argument("--a", type=annot)
+    args = parser.parse_args(["--a", val])
+    assert args.a == expected
+
+
 def test_model_checkpoint_state_key():
     early_stopping = ModelCheckpoint(monitor="val_loss")
     expected_id = (