[feat] Add stronger validation for checkpoint_callback argument

[ananthsub]

What does this PR do?

This removes the option for passing a ModelCheckpoint instance to the checkpoint_callback argument and raises an error if the wrong type is provided.

This argument is already typed as bool in the Trainer, but the callback connector was accepting Union[bool, ModelCheckpoint]. This can be very confusing for users who pass model checkpoint callback instances to this argument, as their callback settings are silently ignored.

This carries forward the deprecation initially landed in #4336

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[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 2021-05-17 17:59:38 UTC

[codecov]

Codecov Report

Merging #7539 (23e92fd) into master (233f252) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7539    +/-   ##
=======================================
- Coverage      92%     88%    -4%     
=======================================
  Files         196     197     +1     
  Lines       12835   12884    +49     
=======================================
- Hits        11832   11318   -514     
- Misses       1003    1566   +563     

[awaelchli]

nice!

[tchaton]

LGMT !