debug failing tests for Fabric with ddp_fork on PT 2.8 -> revert #21057

[Borda]

What does this PR do?

reverting #21057 even on the PR all CI was green

addressing:

FAILED strategies/launchers/test_multiprocessing_integration.py::test_memory_sharing_disabled[ddp_fork] - torch.multiprocessing.spawn.ProcessRaisedException: 

-- Process 1 terminated with the following error:
Traceback (most recent call last):
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/torch/multiprocessing/spawn.py", line 90, in _wrap
    fn(i, *args)
  File "/home/runner/work/pytorch-lightning/pytorch-lightning/src/lightning_fabric/strategies/launchers/multiprocessing.py", line 142, in _wrapping_function
    results = function(*args, **kwargs)
              ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/pytorch-lightning/pytorch-lightning/src/lightning_fabric/fabric.py", line 934, in _wrap_with_setup
    return to_run(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/runner/work/pytorch-lightning/pytorch-lightning/tests/tests_fabric/strategies/launchers/test_multiprocessing_integration.py", line 57, in _test_memory_sharing_disabled
    fabric.barrier()
  File "/home/runner/work/pytorch-lightning/pytorch-lightning/src/lightning_fabric/fabric.py", line 547, in barrier
    self._strategy.barrier(name=name)
  File "/home/runner/work/pytorch-lightning/pytorch-lightning/src/lightning_fabric/strategies/ddp.py", line 162, in barrier
    torch.distributed.barrier()
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/torch/distributed/c10d_logger.py", line 81, in wrapper
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/opt/hostedtoolcache/Python/3.12.11/x64/lib/python3.12/site-packages/torch/distributed/distributed_c10d.py", line 4792, in barrier
    device = torch._C._get_accelerator()
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^

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---

📚 Documentation preview 📚: https://pytorch-lightning--21092.org.readthedocs.build/en/21092/

[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/plugins/collectives/torch_collective.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/plugins/collectives/torch_collective.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/plugins/collectives/torch_collective.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/plugins/collectives/torch_collective.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/plugins/collectives/torch_collective.py, src/lightning/fabric/utilities/distributed.py, tests/tests_fabric/strategies/launchers/test_multiprocessing_integration.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/plugins/collectives/torch_collective.py, src/lightning/fabric/utilities/distributed.py, tests/tests_fabric/strategies/launchers/test_multiprocessing_integration.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/plugins/collectives/torch_collective.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/plugins/collectives/torch_collective.py, src/lightning/fabric/utilities/distributed.py.

---

Thank you for your contribution! 💜

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87%. Comparing base (8d1a734) to head (3688da5).
⚠️ Report is 9 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #21092   +/-   ##
=======================================
- Coverage      87%      87%   -0%     
=======================================
  Files         269      269           
  Lines       23507    23501    -6     
=======================================
- Hits        20495    20487    -8     
- Misses       3012     3014    +2