Fix ModelParallelStrategy fails with non-distributed checkpoint.

[littlebullGit]

What does this PR do?

• Adds a CUDA-only integration test that mirrors the reporter’s compiled ModelParallel setup so the KeyError('model.0.weight') reproduces in CI.
• Fixes [ModelParallelStrategy.optimizer_state] so when torch.compile wraps the module, optimizer states get rekeyed through both the compiled wrapper and the original module before single-file checkpointing, preventing the KeyError.
• Documents the fix in the unreleased changelog.

Fixes #21357

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? (CUDA test runs in CI; CPU run skips as expected)
• 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)

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 79%. Comparing base (f3f6605) to head (ffcd6e4).
⚠️ Report is 1 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (f3f6605) and HEAD (ffcd6e4). Click for more details.

HEAD has 385 uploads less than BASE

Flag	BASE (f3f6605)	HEAD (ffcd6e4)
python3.10	12	3
cpu	119	30
lightning	59	15
pytest	60	0
python3.11	24	6
lightning_fabric	30	0
python3.12	36	9
python3.12.7	35	9
python	12	3
pytorch2.1	12	6
pytest-full	59	30
pytorch2.2.2	6	3
pytorch2.3	6	3
pytorch_lightning	30	15
pytorch2.7	6	3
pytorch2.9	6	3
pytorch2.8	6	3
pytorch2.5.1	6	3
pytorch2.4.1	5	3
pytorch2.6	6	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21384     +/-   ##
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- Coverage      87%      79%     -8%     
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  Files         269      266      -3     
  Lines       23813    23784     -29     
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- Hits        20628    18746   -1882     
- Misses       3185     5038   +1853