Fix ModelCheckpoint.CHECKPOINT_NAME_LAST test interaction (#18993)

(cherry picked from commit b4605b4)

Author: awaelchli

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -485,12 +485,12 @@ def test_model_checkpoint_file_extension(tmpdir):
     assert set(expected) == set(os.listdir(tmpdir))
 
 
-def test_model_checkpoint_save_last(tmpdir):
+def test_model_checkpoint_save_last(tmpdir, monkeypatch):
     """Tests that save_last produces only one last checkpoint."""
     seed_everything()
     model = LogInTwoMethods()
     epochs = 3
-    ModelCheckpoint.CHECKPOINT_NAME_LAST = "last-{epoch}"
+    monkeypatch.setattr(ModelCheckpoint, "CHECKPOINT_NAME_LAST", "last-{epoch}")
     model_checkpoint = ModelCheckpoint(monitor="early_stop_on", dirpath=tmpdir, save_top_k=-1, save_last=True)
     trainer = Trainer(
         default_root_dir=tmpdir,
@@ -511,7 +511,6 @@ def test_model_checkpoint_save_last(tmpdir):
     )
     assert os.path.islink(tmpdir / last_filename)
     assert os.path.realpath(tmpdir / last_filename) == model_checkpoint._last_checkpoint_saved
-    ModelCheckpoint.CHECKPOINT_NAME_LAST = "last"
 
 
 def test_model_checkpoint_link_checkpoint(tmp_path):