debug failing tests for Fabric with ddp_fork on PT 2.8

[Borda]

What does this PR do?

This needs have a permanent resolution

• Right now, on master, it is a very flaky test, see bump: try deepspeed >=0.14.1,<=0.15.0 #21076 (comment)
• When we merged let _get_default_process_group_backend_for_device support more hardware platforms #21057 it was failing constantly
• Direction is to properly init the group

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📚 Documentation preview 📚: https://pytorch-lightning--21093.org.readthedocs.build/en/21093/

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu-guardian	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

🟢 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs) (testing Lightning | latest)	success	✅
pytorch-lightning (GPUs) (testing PyTorch | oldest)	success	✅
pytorch-lightning (GPUs) (testing PyTorch | latest)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

🟢 pytorch_lightning: Benchmarks

Check ID	Status
lightning.Benchmarks	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

🟢 fabric: Docs

Check ID	Status
docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

🟢 lightning_fabric: CPU workflow

Check ID	Status
fabric-cpu-guardian	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py, tests/tests_fabric/utilities/test_distributed.py.

🟢 lightning_fabric: Azure GPU

Check ID	Status
lightning-fabric (GPUs) (testing Fabric | oldest)	success	✅
lightning-fabric (GPUs) (testing Fabric | latest)	success	✅
lightning-fabric (GPUs) (testing Lightning | latest)	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py, tests/tests_fabric/utilities/test_distributed.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

🟢 install

Check ID	Status
install-pkg-guardian	success	✅

These checks are required after the changes to src/lightning/fabric/strategies/ddp.py, src/lightning/fabric/utilities/distributed.py.

---

Thank you for your contribution! 💜

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[codecov]

Codecov Report

❌ Patch coverage is 27.27273% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 87%. Comparing base (663b6ce) to head (f9a6728).
⚠️ Report is 5 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21093   +/-   ##
=======================================
- Coverage      87%      87%   -0%     
=======================================
  Files         269      269           
  Lines       23595    23604    +9     
=======================================
- Hits        20582    20581    -1     
- Misses       3013     3023   +10