Fix LR scheduler behaviour with AMP

[milesial]

What does this PR do?

When training when native AMP and a LR scheduler, we get this warning that indicates that a LR step has been taken when an optimizer step was skipped (expected at the beginning of the training with native AMP):

/usr/local/lib/python3.8/dist-packages/torch/optim/lr_scheduler.py:138: UserWarning: Detected call of `lr_scheduler.step()` before `optimizer.step()`. In PyTorch 1.1.0 and later, you should call them in the opposite order: `optimizer.step()` before `lr_scheduler.step()`.  Failure to do this will result in PyTorch skipping the first value of the learning rate schedule. See more details at https://pytorch.org/docs/stable/optim.html#how-to-adjust-learning-rate

Fixes #16228 #5558

Does your PR introduce any breaking changes? If yes, please list them.

No

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[milesial]

In the process of fixing tests I discovered and fixed a bug where the scheduler wouldn't match its optimizer when multiple optimizers are instantiated with frequencies. Now the optimizers and schedulers match and alternate as they should, resetting the cycle every epoch.

[milesial]

@carmocca Ready for final review

[milesial]

The way I fixed the tests/tests_pytorch/models/test_hooks.py::test_trainer_model_hook_system_fit[True-kwargs1] tests is very flaky, so I'd appreciate if someone more familiar with these tests comes up with a better fix. edit: seems like it didn't even fix it...

[milesial]

I also modified native_amp.py in both lightning_fabric and pytorch_lightning. It doesn't seem like the lightning_fabric one is called for a typical workflow, so I don't know if it's the right way

[mrembalski]

Hi @Borda, I also encounter the same issue. Will this be merged?

[Borda]

Hi @Borda, I also encounter the same issue. Will this be merged?

Let me check what is missing here...

[alimoezzi]

Is this PR merged already? I'm still having this issue.

[Borda]

there were some failing tests, @milesial mind have a look?

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[codecov[bot]]

Codecov Report

Merging #16229 (965fc03) into master (6497e36) will decrease coverage by 54%. Report is 1 commits behind head on master. The diff coverage is 29%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #16229      +/-   ##
==========================================
- Coverage      83%      29%     -54%     
==========================================
  Files         450      442       -8     
  Lines       38089    37941     -148     
==========================================
- Hits        31803    11015   -20788     
- Misses       6286    26926   +20640