Add EMAWeightAveraging callback to weight_averaging.py

[amorehead]

What does this PR do?

Adds a default EMAWeightAveraging callback to weight_averaging.py for convenience.
Now, users of weight_averaging.py can directly instantiate an EMA callback for model training with safe default hyperparameters chosen.

Before submitting

• Was this discussed/agreed 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 list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

Anyone in the community is welcome to review the PR.
Before you start reviewing, make sure you have read the review guidelines. In short, see the following bullet-list:

Reviewer checklist

• 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

---

📚 Documentation preview 📚: https://pytorch-lightning--21260.org.readthedocs.build/en/21260/

[deependujha]

Hi @amorehead , can you please write tests for it?

[amorehead]

Hi, @deependujha. I've added a few unit tests as requested.

[amorehead]

@Borda, (nearly) all tests are passing. Are we OK with merging this PR as is?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79%. Comparing base (126fa6f) to head (de0aa2d).
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (126fa6f) and HEAD (de0aa2d). Click for more details.

HEAD has 660 uploads less than BASE

Flag	BASE (126fa6f)	HEAD (de0aa2d)
cpu	180	30
lightning_fabric	45	0
pytest	90	0
python3.12	54	9
python3.10	18	3
lightning	90	15
python3.12.7	54	9
python3.11	36	6
python	18	3
pytorch2.1	18	6
pytest-full	90	30
pytorch_lightning	45	15
pytorch2.6	9	3
pytorch2.8	9	3
pytorch2.7	9	3
pytorch2.9	9	3
pytorch2.3	9	3
pytorch2.5.1	9	3
pytorch2.4.1	9	3
pytorch2.2.2	9	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21260     +/-   ##
=========================================
- Coverage      87%      79%     -8%     
=========================================
  Files         269      266      -3     
  Lines       23745    23707     -38     
=========================================
- Hits        20570    18689   -1881     
- Misses       3175     5018   +1843