Fix ModelCheckpoint file_exists OOM in DDP

[littlebullGit]

What does this PR do?

• Use [strategy.reduce_boolean_decision] instead of [broadcast] in [ModelCheckpoint.file_exists].
• Ensure only global rank 0 touches the filesystem when checking for existing checkpoints.
• Avoid broadcast_object_list for a simple boolean in DDP to reduce memory pressure in the checkpoint path.
• Add a small DDP test with monitor=None to exercise this path.

Fixes #19674

Motivation and context

In DDP, [strategy.broadcast] is implemented via torch.distributed.broadcast_object_list, which serializes the Python object and can allocate unnecessary GPU memory even for a single boolean. For the “file exists” decision we only need a tiny boolean reduction, so [reduce_boolean_decision] is a better fit and addresses the CUDA OOM reported in #19674 while preserving behavior.

Dependencies

• No new runtime dependencies introduced by this PR.
• Tests rely on pytorch_lightning_enterprise being available, as required by [tests/tests_pytorch/conftest.py].

Tests

All run inside the project .venv:

• python -m pytest tests/tests_pytorch/checkpointing/test_checkpoint_callback_frequency.py
• python -m pytest tests/tests_pytorch/checkpointing -k "not legacy_checkpoints"
• python -m pytest tests/tests_pytorch/callbacks/test_model_checkpoint_*.py tests/tests_pytorch/trainer/test_trainer.py

---

📚 Documentation preview 📚: https://pytorch-lightning--21380.org.readthedocs.build/en/21380/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 82%. Comparing base (8f702b3) to head (58d8c50).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

❗ There is a different number of reports uploaded between BASE (8f702b3) and HEAD (58d8c50). Click for more details.

HEAD has 1451 uploads less than BASE

Flag	BASE (8f702b3)	HEAD (58d8c50)
cpu	355	29
lightning_fabric	89	0
pytest	179	0
python3.12	108	9
python3.12.7	106	9
python3.10	36	2
lightning	179	14
python3.11	72	6
python	33	3
pytorch2.2.2	18	3
pytest-full	176	29
pytorch2.4.1	17	3
pytorch2.3	18	3
pytorch2.1	34	5
pytorch2.9	18	3
pytorch_lightning	87	15
pytorch2.7	18	3
pytorch2.5.1	18	3
pytorch2.8	18	3
pytorch2.6	17	3

Additional details and impacted files

@@            Coverage Diff            @@
##           master   #21380     +/-   ##
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- Coverage      89%      82%     -7%     
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  Files         269      266      -3     
  Lines       22050    22008     -42     
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- Hits        19727    18074   -1653     
- Misses       2323     3934   +1611     

[justusschock]

Great job @littlebullGit ,

One minor comment. Could you also please add a changelog entry?