Deprecate save_function from model checkpoint callback

[ananthsub]

What does this PR do?

Fixes #3875

This is minor cleanup to avoid the indirection from the trainer and checkpoint callback. Enforcing that saving goes through the trainer will avoid potential bugs for items like deepspeed or sharded model saving.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• 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)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

Did you have fun?

Make sure you had fun coding 🙃

[codecov]

Codecov Report

Merging #7201 (03b1266) into master (68eac4d) will decrease coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7201    +/-   ##
=======================================
- Coverage      87%     87%    -0%     
=======================================
  Files         198     198            
  Lines       12632   12733   +101     
=======================================
+ Hits        10988   11051    +63     
- Misses       1644    1682    +38     

[carmocca]

Should we deprecate this first? Override the save_function setter and show a warning

[ananthsub]

I'll mark this as deprecated on the callback instead of removing it outright in this PR

[pep8speaks]

Hello @ananthsub! Thanks for updating this PR.

There are currently no PEP 8 issues detected in this Pull Request. Cheers! 🍻

Comment last updated at 2021-04-26 16:26:16 UTC

[tchaton]

LGTM