Implement test to reproduce the issue

Author: baskrahmer

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -1666,3 +1666,30 @@ def val_dataloader(self) -> DataLoader:
     trainer_kwargs["max_epochs"] = 4
     trainer = Trainer(**trainer_kwargs, callbacks=ModelCheckpoint(**mc_kwargs))
     trainer.fit(model, ckpt_path=checkpoint_path)
+
+
+def test_save_last_without_save_on_train_epoch_and_without_val(tmp_path):
+    """Test that save_last=True when save_on_train_epoch_end=False"""
+
+    # Remove validation methods to reproduce the bug
+    model = BoringModel()
+    model.validation_step = None
+    model.val_dataloader = None
+    
+    checkpoint_callback = ModelCheckpoint(
+        dirpath=tmp_path,
+        save_last=True,
+        save_on_train_epoch_end=False,
+    )
+    
+    trainer = Trainer(
+        max_epochs=2,
+        callbacks=[checkpoint_callback],
+        logger=False,
+        enable_progress_bar=False,
+    )
+    
+    trainer.fit(model)
+    
+    # save_last=True should always save last.ckpt
+    assert (tmp_path / "last.ckpt").exists()