Replace a MisconfigurationException with warning in ModelCheckpoint c…

…allback (#4560)

* replace MisconfigurationException with warning

* update test

* check raising UserWarning

Co-authored-by: chaton <thomas@grid.ai>
Co-authored-by: Nicki Skafte <skaftenicki@gmail.com>
(cherry picked from commit 30ad3e2)

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