fix plateau scheduler stepping on incomplete epoch

[awaelchli]

What does this PR do?

Fixes #8839

Added test FAILS without the fix applied.

This fixes an issue with the training loop trying to step an epoch level scheduler when the epoch is incomplete. This happens in the case when e.g. we specify Trainer(max_steps=x) where x < one epoch. This usually does not matter, but is problematic when the scheduler monitors a metric from a validation epoch, which does not actually run if we finish training early.

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?

I made sure I had fun coding 🙃

[codecov]

Codecov Report

Merging #8861 (f08941e) into master (fec4f28) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #8861    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         169     169            
  Lines       14078   14079     +1     
=======================================
- Hits        13052   12475   -577     
- Misses       1026    1604   +578