Update LR step scheduler to use total step to work across epochs

[falckt]

What does this PR do?

Fixes #17544

I am not sure whether to consider this as a breaking change. For frequencies > epoch length I would consider it a bug fix. For frequencies < epoch length it does change the behavior but IMHO not in a fundamental way and personally I expected the new behavior to begin with.

Before submitting

• Was this discussed/agreed via a GitHub issue? change according to @awaelchli suggestion in Stepwise LR-Scheduler not working across epochs  #17544 (comment)

• 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?

  I don't think it is necessary, I would consider the new behavior the expected one.

• Did you write any new necessary tests?

  I couldn't find any tests for the old code. Q: does this need a test?

• Did you verify new and existing tests pass locally with your changes? it works as expected locally

• Did you list all the breaking changes introduced by this pull request? yes

• Did you update the CHANGELOG? todo

PR review

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📚 Documentation preview 📚: https://pytorch-lightning--20248.org.readthedocs.build/en/20248/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (474bdd0) to head (fd1883f).
Report is 1 commits behind head on master.

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HEAD has 87 uploads less than BASE

Flag	BASE (474bdd0)	HEAD (fd1883f)
cpu	41	21
python3.10	12	6
lightning_fabric	7	0
pytest	25	2
lightning	31	16
python3.9	11	6
gpu	4	2
python3.11	12	6
python3.12	6	3

Additional details and impacted files

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##           master   #20248     +/-   ##
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- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23064    23009     -55     
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- Hits        20573    18606   -1967     
- Misses       2491     4403   +1912