test: flaky terminated with signal SIGABRT

[kaiyaok2]

What does this PR do?

Fixes #20620

The test tests/tests_pytorch/core/test_results.py::test_result_reduce_ddp fails intermittently, similar to the test addressed in #20537, with the error:

torch.multiprocessing.spawn.ProcessExitedException: process 0 terminated with signal SIGABRT

To address this, the test could be marked flaky.

Background

We are evaluating how our tool for test prioritization could find test failures faster in your project. During a CI rerun for commit 7322d63bef2cf1a0439f8b19b545cd4a89da62b0, we encountered the failure of this test, which you can see in this log: GitHub Actions Log.

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📚 Documentation preview 📚: https://pytorch-lightning--20621.org.readthedocs.build/en/20621/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79%. Comparing base (1f5add3) to head (0f81eee).
Report is 117 commits behind head on master.

❗ There is a different number of reports uploaded between BASE (1f5add3) and HEAD (0f81eee). Click for more details.

HEAD has 2689 uploads less than BASE

Flag	BASE (1f5add3)	HEAD (0f81eee)
gpu	2	0
pytest	314	0
lightning	468	18
cpu	623	24
python3.9	156	6
lightning_fabric	79	0
python3.10	78	3
python3.11	155	6
python3.12.7	234	9
pytorch2.1	117	9
pytest-full	311	24
pytorch_lightning	78	6
pytorch2.3	38	3
pytorch2.2.2	39	3
pytorch2.4.1	39	3
pytorch2.5.1	78	6

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #20621     +/-   ##
=========================================
- Coverage      88%      79%     -9%     
=========================================
  Files         267      264      -3     
  Lines       23366    23311     -55     
=========================================
- Hits        20475    18364   -2111     
- Misses       2891     4947   +2056