Stepwise LR scheduler

[01AbhiSingh]

What does this PR do?

Fixes #<17544>

Hii @awaelchli. Can you please verify the changes I made. If they are correct then i will take up and correct any failing tests also.

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📚 Documentation preview 📚: https://pytorch-lightning--20211.org.readthedocs.build/en/20211/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81%. Comparing base (5be58f6) to head (3c48c9e).
Report is 14 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (5be58f6) and HEAD (3c48c9e). Click for more details.

HEAD has 553 uploads less than BASE

Flag	BASE (5be58f6)	HEAD (3c48c9e)
cpu	147	21
lightning	106	16
pytest	87	2
python3.9	43	6
python3.10	42	6
lightning_fabric	25	0
gpu	4	2
python3.11	42	6
python3.12	20	3
pytorch2.1	38	12
pytest-full	64	21
pytorch2.3	9	3
pytorch_lightning	20	7
pytorch2.2	9	3
pytorch2.4	8	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20211     +/-   ##
=========================================
- Coverage      89%      81%     -8%     
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  Files         267      264      -3     
  Lines       23084    23029     -55     
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- Hits        20585    18618   -1967     
- Misses       2499     4411   +1912     

[01AbhiSingh]

Hii @Borda. Do I need to make any kind of changes in the PR ?

[lantiga]

This looks good, thank you for the contribution @01AbhiSingh

Ideally we could add a test to verify the behavior described in #17544. The current test suite can't detect the current change and this is usually a sign of insufficient coverage. Would you be willing to contribute such test?

[01AbhiSingh]

Yes, sure let me look into it.

[01AbhiSingh]

Hi @lantiga , Do you want a new test written from scratch or need me to make the necessary changes in a preexisting file? All the tests have been passed. If the changes need to be made in a preexisting file, it would be very helpful if you could point out the test in which I need to make the changes, as all the tests have been passed, and due to that, I can't find the test.

[lantiga]

hey @01AbhiSingh sorry for the wait

You can take inspiration from:

pytorch-lightning/tests/tests_pytorch/trainer/optimization/test_optimizers.py

Line 459 in b0aa504

def test_lr_scheduler_epoch_step_frequency(mocked_sched, check_val_every_n_epoch, tmp_path):

and add a new test where scheduling goes across epoch boundaries. Maybe @falckt can help too?

[lantiga]

Hey @01AbhiSingh will you have time to add a test?

[01AbhiSingh]

Hey @01AbhiSingh will you have time to add a test?

Yes, Today's my last exam after that I will add the test asap. Sorry for the delay.

[lantiga]

@01AbhiSingh just checking, no pressure :-)