Fix save_weights_only flag in ModelCheckpoint

[kepler]

ModelCheckpoint then passes self.save_weights_only to the save function.

Before submitting

• Was this discussed/approved via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?
• If you made a notable change (that affects users), did you update the CHANGELOG?

What does this PR do?

Fixes #427.

PR review

Anyone in the community is free to review the PR once the tests have passed.

[kepler]

I patched the dump_checkpoint and save_checkpoint methods in TrainerIOMixin to include a weights_only flag, which defaults to False and so should be backwards compatible.

One alternative would be to read the flag from self.checkpoint_callback inside dump_checkpoint, but I find it sub-optimal (for a few reasons).

What's missing is verifying the loading side of things, regarding whether some error handling is necessary.

[Borda]

the GH diff is strange it makes the same code as change..

[mergify]

This pull request is now in conflict... :(

[codecov]

Codecov Report

Merging #1780 into master will increase coverage by 0%.
The diff coverage is 95%.

@@          Coverage Diff           @@
##           master   #1780   +/-   ##
======================================
  Coverage      88%     88%           
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  Files          71      71           
  Lines        4458    4461    +3     
======================================
+ Hits         3915    3918    +3     
  Misses        543     543           

[williamFalcon]

@kepler thanks, this is great!