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
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• 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)

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