Fix save_last type annotation for ModelCheckpoint (#19808)

Author: mariovas3

src/lightning/pytorch/CHANGELOG.md

@@ -57,7 +57,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 
 - Fixed `WandbLogger.log_hyperparameters()` raising an error if hyperparameters are not JSON serializable ([#19769](https://github.com/Lightning-AI/pytorch-lightning/pull/19769))
 
--
+
+- Fixed an issue with the LightningCLI not being able to set the `ModelCheckpoint(save_last=...)` argument ([#19808](https://github.com/Lightning-AI/pytorch-lightning/pull/19808))
 
 
 ## [2.2.2] - 2024-04-11

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
@@ -1601,3 +1603,24 @@ def test_expand_home():
     # it is possible to have a folder with the name `~`
     checkpoint = ModelCheckpoint(dirpath="./~/checkpoints")
     assert checkpoint.dirpath == str(Path.cwd() / "~" / "checkpoints")
+
+
+@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):
+    """Test that the CLI can parse the `save_last` argument correctly (composed type)."""
+    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