Replace a MisconfigurationException with warning in ModelCheckpoint callback (#4560)

* replace MisconfigurationException with warning

* update test

* check raising UserWarning

Co-authored-by: chaton <[email protected]>
Co-authored-by: Nicki Skafte <[email protected]>
(cherry picked from commit 30ad3e2)

Author: 3 people

pytorch_lightning/callbacks/model_checkpoint.py

@@ -247,9 +247,9 @@ def __validate_init_configuration(self):
                     ' configuration. No quantity for top_k to track.'
                 )
             if self.save_last:
-                raise MisconfigurationException(
-                    'ModelCheckpoint(save_last=True, monitor=None) is not a valid configuration.'
-                    ' You can save the last checkpoint with ModelCheckpoint(save_top_k=None, monitor=None)'
+                rank_zero_warn(
+                    'ModelCheckpoint(save_last=True, monitor=None) is a redundant configuration.'
+                    ' You can save the last checkpoint with ModelCheckpoint(save_top_k=None, monitor=None).'
                 )
 
     def __init_ckpt_dir(self, filepath, dirpath, filename, save_top_k):

tests/checkpointing/test_model_checkpoint.py

@@ -284,8 +284,8 @@ def test_none_monitor_top_k(tmpdir):
 
 def test_none_monitor_save_last(tmpdir):
     """ Test that a warning appears for save_last=True with monitor=None. """
-    with pytest.raises(
-        MisconfigurationException, match=r'ModelCheckpoint\(save_last=True, monitor=None\) is not a valid.*'
+    with pytest.warns(
+        UserWarning, match=r'ModelCheckpoint\(save_last=True, monitor=None\) is a redundant.*'
     ):
         ModelCheckpoint(dirpath=tmpdir, save_last=True)
     # These should not fail