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)
• [x] Did you read the contributor guideline, Pull Request section?
• [x] 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)
• [x] 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?
• [x] Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

[codecov[bot]]

Codecov Report

:x: Patch coverage is 92.30769% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 79%. Comparing base (79ffe50) to head (2e5ae5f). :white_check_mark: All tests successful. No failed tests found.

:exclamation: There is a different number of reports uploaded between BASE (79ffe50) and HEAD (2e5ae5f). Click for more details.

HEAD has 925 uploads less than BASE

Flag	BASE (79ffe50)	HEAD (2e5ae5f)
cpu	239	30
lightning_fabric	60	0
pytest	120	0
python3.12	71	9
python3.12.7	72	9
lightning	120	15
python3.11	48	6
python3.10	24	3
python	24	3
pytorch2.1	24	6
pytest-full	119	30
pytorch_lightning	59	15
pytorch2.6	12	3
pytorch2.4.1	12	3
pytorch2.3	12	3
pytorch2.2.2	12	3
pytorch2.5.1	12	3
pytorch2.9	12	3
pytorch2.7	12	3
pytorch2.8	11	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       23804    23775     -29     
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- Hits        20626    18752   -1874     
- Misses       3178     5023   +1845