Force ModelCheckpoint callback to run last

[awaelchli]

What does this PR do?

Force ModelCheckpoint callbacks to run last, ensuring that all states get saved to the checkpoint.

The test that I added fail if we remove the changes in the callback connector.
This change only affects usecases where the user provides their own callback list.
The automated default ModelCheckpoint was already running last before this PR.

PR #5615 will make use of this helper function and add more tests to ensure the same applies to the model checkpoints defined by configure_callbacks.

Before submitting

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[ananthsub]

Thanks for sending this out so quickly @awaelchli ! Any reason this is blocked on #5621? Can this go in first and then be refactored by #5621?

[codecov]

Codecov Report

Merging #5731 (bc11711) into release/1.2-dev (aba2123) will decrease coverage by 0%.
The diff coverage is 100%.

@@               Coverage Diff                @@
##           release/1.2-dev   #5731    +/-   ##
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- Coverage               88%     88%    -0%     
================================================
  Files                  181     181            
  Lines                12890   12747   -143     
================================================
- Hits                 11339   11195   -144     
- Misses                1551    1552     +1     

[pep8speaks]

Hello @awaelchli! Thanks for updating this PR.

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

Comment last updated at 2021-02-03 14:38:18 UTC

[awaelchli]

@ananthsub No particular reason, just a matter of rebasing. I changed it now

[tchaton]

LGTM !