zyaoj/harden lr scheduler

[zyaoj]

What does this PR do? Please describe:

• Fix mypy bypass for cosine annealing lr scheduler
• Add unit tests

Test coverage before this PR:

---------- coverage: platform linux, python 3.10.14-final-0 ----------
Name                                                       Stmts   Miss  Cover   Missing
----------------------------------------------------------------------------------------
src/fairseq2/optim/lr_scheduler/factory.py                   104     31    70%   49-51, 84-113, 139, 154, 182-191, 222-231

Test coverage after this PR:

---------- coverage: platform linux, python 3.10.14-final-0 ----------
Name                                                       Stmts   Miss  Cover   Missing
----------------------------------------------------------------------------------------
src/fairseq2/optim/lr_scheduler/factory.py                   104      5    95%   49-51, 109, 139, 154

Does your PR introduce any breaking changes? If yes, please list them:
N/A

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that 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?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)

[cbalioglu]

Nice! Thanks for the improvements and tests!