Add strategy="auto" support on the 1.9.x branch

[carmocca]

What does this PR do?

This adds support for passing strategy="auto" to Fabric or Trainer. This is essentially the same as the previous strategy=None behavior, but will favor ddp over ddp_spawn when possible. strategy=None is still the default.

cc @Borda @tchaton @carmocca @justusschock @awaelchli

[github-actions]

⛈️ Required checks status: Has failure 🔴

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Groups summary

🔴 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu (macOS-11, pytorch, 3.8, 1.11)	failure	❌
pl-cpu (macOS-11, pytorch, 3.9, 1.12)	failure	❌
pl-cpu (macOS-11, pytorch, 3.10, 1.13)	failure	❌
pl-cpu (macOS-11, pytorch, 3.8, 1.10, oldest)	failure	❌
pl-cpu (ubuntu-20.04, pytorch, 3.8, 1.10)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.9, 1.11)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.10, 1.12)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.10, 1.13)	success	✅
pl-cpu (ubuntu-20.04, pytorch, 3.7, 1.10, oldest)	success	✅
pl-cpu (windows-2022, pytorch, 3.9, 1.11)	success	✅
pl-cpu (windows-2022, pytorch, 3.10, 1.12)	success	✅
pl-cpu (windows-2022, pytorch, 3.10, 1.13)	success	✅
pl-cpu (windows-2022, pytorch, 3.7, 1.10, oldest)	success	✅
pl-cpu (slow, macOS-11, pytorch, 3.7, 1.11)	failure	❌
pl-cpu (slow, ubuntu-20.04, pytorch, 3.7, 1.11)	success	✅
pl-cpu (slow, windows-2022, pytorch, 3.7, 1.11)	success	✅
pl-cpu (macOS-11, lightning, 3.8, 1.13)	failure	❌
pl-cpu (ubuntu-20.04, lightning, 3.8, 1.13)	success	✅
pl-cpu (windows-2022, lightning, 3.8, 1.13)	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, src/pytorch_lightning/trainer/connectors/accelerator_connector.py, tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py.

🟢 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs)	success	✅

These checks are required after the changes to src/pytorch_lightning/trainer/connectors/accelerator_connector.py, tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py, src/lightning_fabric/connector.py.

🟢 pytorch_lightning: Azure HPU

Check ID	Status
pytorch-lightning (HPUs)	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, src/pytorch_lightning/trainer/connectors/accelerator_connector.py, tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py.

🟡 pytorch_lightning: Azure IPU

Check ID	Status
pytorch-lightning (IPUs)	queued	⌛

These checks are required after the changes to src/lightning_fabric/connector.py, src/pytorch_lightning/trainer/connectors/accelerator_connector.py, tests/tests_pytorch/trainer/connectors/test_accelerator_connector.py.

🟢 pytorch_lightning: Docs

Check ID	Status
make-doctest (pytorch)	success	✅
make-html (pytorch)	success	✅

These checks are required after the changes to src/pytorch_lightning/trainer/connectors/accelerator_connector.py.

🟢 lightning_fabric: CPU workflow

Check ID	Status
fabric-cpu (macOS-11, fabric, 3.8, 1.11)	success	✅
fabric-cpu (macOS-11, fabric, 3.9, 1.12)	success	✅
fabric-cpu (macOS-11, fabric, 3.10, 1.13)	success	✅
fabric-cpu (macOS-11, fabric, 3.7, 1.10, oldest)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.8, 1.10)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.9, 1.11)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.10, 1.12)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.10, 1.13)	success	✅
fabric-cpu (ubuntu-20.04, fabric, 3.7, 1.10, oldest)	success	✅
fabric-cpu (windows-2022, fabric, 3.9, 1.11)	success	✅
fabric-cpu (windows-2022, fabric, 3.10, 1.12)	success	✅
fabric-cpu (windows-2022, fabric, 3.10, 1.13)	success	✅
fabric-cpu (windows-2022, fabric, 3.7, 1.10, oldest)	success	✅
fabric-cpu (macOS-11, lightning, 3.8, 1.13)	success	✅
fabric-cpu (ubuntu-20.04, lightning, 3.8, 1.13)	success	✅
fabric-cpu (windows-2022, lightning, 3.8, 1.13)	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, tests/tests_fabric/test_connector.py.

🟢 lightning_fabric: Azure GPU

Check ID	Status
lightning-fabric (GPUs)	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, tests/tests_fabric/test_connector.py.

🟢 mypy

Check ID	Status
mypy	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, src/pytorch_lightning/trainer/connectors/accelerator_connector.py.

🟢 install

Check ID	Status
install-pkg (ubuntu-22.04, app, 3.7)	success	✅
install-pkg (ubuntu-22.04, app, 3.10)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.7)	success	✅
install-pkg (ubuntu-22.04, fabric, 3.10)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.7)	success	✅
install-pkg (ubuntu-22.04, pytorch, 3.10)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.7)	success	✅
install-pkg (ubuntu-22.04, lightning, 3.10)	success	✅
install-pkg (ubuntu-22.04, notset, 3.7)	success	✅
install-pkg (ubuntu-22.04, notset, 3.10)	success	✅
install-pkg (macOS-12, app, 3.7)	success	✅
install-pkg (macOS-12, app, 3.10)	success	✅
install-pkg (macOS-12, fabric, 3.7)	success	✅
install-pkg (macOS-12, fabric, 3.10)	success	✅
install-pkg (macOS-12, pytorch, 3.7)	success	✅
install-pkg (macOS-12, pytorch, 3.10)	success	✅
install-pkg (macOS-12, lightning, 3.7)	success	✅
install-pkg (macOS-12, lightning, 3.10)	success	✅
install-pkg (macOS-12, notset, 3.7)	success	✅
install-pkg (macOS-12, notset, 3.10)	success	✅
install-pkg (windows-2022, app, 3.7)	success	✅
install-pkg (windows-2022, app, 3.10)	success	✅
install-pkg (windows-2022, fabric, 3.7)	success	✅
install-pkg (windows-2022, fabric, 3.10)	success	✅
install-pkg (windows-2022, pytorch, 3.7)	success	✅
install-pkg (windows-2022, pytorch, 3.10)	success	✅
install-pkg (windows-2022, lightning, 3.7)	success	✅
install-pkg (windows-2022, lightning, 3.10)	success	✅
install-pkg (windows-2022, notset, 3.7)	success	✅
install-pkg (windows-2022, notset, 3.10)	success	✅

These checks are required after the changes to src/lightning_fabric/connector.py, src/pytorch_lightning/trainer/connectors/accelerator_connector.py.

🔴 link-check

Check ID	Status
markdown-link-check	failure	❌

These checks are required after the changes to src/lightning_app/CHANGELOG.md, src/lightning_fabric/CHANGELOG.md, src/pytorch_lightning/CHANGELOG.md.

---

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[codecov]

Codecov Report

Merging #16916 (39bdcfc) into release/stable (1091484) will decrease coverage by 20%.
The diff coverage is 100%.

❗ Current head 39bdcfc differs from pull request most recent head c02898e. Consider uploading reports for the commit c02898e to get more accurate results

Additional details and impacted files

@@                Coverage Diff                @@
##           release/stable   #16916     +/-   ##
=================================================
- Coverage              82%      62%    -20%     
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  Files                 474      435     -39     
  Lines               35160    34979    -181     
=================================================
- Hits                28685    21654   -7031     
- Misses               6475    13325   +6850