updated model_checkpoint.py to add the facility of retaining periodic checkpoints

[arijit-hub]

… checkpoints

What does this PR do?

This decouples the save_top_k from saving intermediate checkpoints when setting every_n_epochs or every_n_steps to some value. This is a direct solution to close #20539 and further motivation can be gathered from the thread's discussion.

• [x]Was this discussed/agreed via a GitHub issue? (not for typos and docs) : Yes. See top_k parameter of ModelCheckpoint default value #20539
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs): NO. It just add a new condition and seems to work.
• Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

@Borda @lantiga

[lantiga]

Thank you @arijit-hub, we do need a test that goes with it.
The test should verify that we remove previous checkpoints on given conditions, and that backward compatibility is guaranteed.

Take a look here as a starting point:
https://github.com/Lightning-AI/pytorch-lightning/blob/master/tests/tests_pytorch/checkpointing/test_model_checkpoint.py
https://github.com/Lightning-AI/pytorch-lightning/blob/master/tests/tests_pytorch/checkpointing/test_checkpoint_callback_frequency.py

I'd be great if you could give it a shot, if not please let me know. Thanks for your contribution!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (a944e77) to head (3bc6e9f).
Report is 1 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (a944e77) and HEAD (3bc6e9f). Click for more details.

HEAD has 5427 uploads less than BASE

Flag	BASE (a944e77)	HEAD (3bc6e9f)
cpu	1228	24
lightning	922	18
pytest	601	0
python3.9	308	6
lightning_fabric	151	0
python3.10	153	3
python3.11	309	6
python3.12.7	458	9
gpu	2	0
pytorch2.1	236	9
pytest-full	629	24
pytorch_lightning	157	6
pytorch2.2.2	79	3
pytorch2.3	80	3
pytorch2.4.1	78	3
pytorch2.5.1	156	6

Additional details and impacted files

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- Misses       2899     4959   +2060