Make subprocess launcher the default in Lite

[awaelchli]

What does this PR do?

Changes the default launcher when strategy=None (not specified by user).

fabric = Fabric(strategy=None, devices>1)
fabric.launch()

Before: This selected the "ddp_spawn" setting.
Now: This selects the "ddp" setting.

This aligns it with the lightning run model CLI command which can't function with the option "ddp_spawn" anyways.
DDP-spawn remains useful for debugging, alongside with DDP-fork in notebooks.

Does your PR introduce any breaking changes? If yes, please list them.

Fabric is in beta.

Before submitting

• Was this discussed/approved via a GitHub issue? (not for typos and docs)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests? (not for typos and docs)
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is free to review the PR once the tests have passed.
Before you start reviewing make sure you have read Review guidelines. In short, see the following bullet-list:

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
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Did you have fun?

I made sure I had fun coding 🙃

cc @Borda @carmocca @justusschock @awaelchli

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Tests workflow

Check ID	Status
pl-cpu (macOS-11, pytorch, 3.8, 1.11)	success	✅
pl-cpu (macOS-11, pytorch, 3.9, 1.12)	success	✅
pl-cpu (macOS-11, pytorch, 3.10, 1.13)	success	✅
pl-cpu (macOS-11, pytorch, 3.8, 1.10, oldest)	success	✅
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)	success	✅
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)	success	✅
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.

🟢 pytorch_lightning: Azure GPU

Check ID	Status
pytorch-lightning (GPUs)	success	✅

These checks are required after the changes to 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.

🟢 pytorch_lightning: Azure IPU

Check ID	Status
pytorch-lightning (IPUs)	success	✅

These checks are required after the changes to src/lightning_fabric/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.

🟢 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.

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Thank you for your contribution! 💜

Note
This comment is automatically generated and updates for 60 minutes every 180 seconds. If you have any other questions, contact carmocca for help.

[carmocca]

Great!