enable import lightning.app if it is installed

[Borda]

What does this PR do?

Enables continues using lightning.app if it is installed and test it

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• Was this discussed/agreed via a GitHub issue? (not for typos and docs)
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📚 Documentation preview 📚: https://pytorch-lightning--20059.org.readthedocs.build/en/20059/

[github-actions]

🌩️ Required checks status: Pending 🟡

Groups summary

🟡 lightning_fabric: Azure GPU

Check ID	Status
lightning-fabric (GPUs) (testing Fabric	latest)	no_status
lightning-fabric (GPUs) (testing Lightning	latest)	no_status

These checks are required after the changes to examples/fabric/tensor_parallel/train.py.

🟡 mypy

Check ID	Status
mypy	no_status	❓

These checks are required after the changes to requirements/app/app.txt, src/lightning/app/__init__.py.

🟡 install

Check ID	Status
install-pkg (ubuntu-22.04, fabric, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, fabric, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, pytorch, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, pytorch, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, lightning, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, lightning, 3.11)	no_status	❓
install-pkg (ubuntu-22.04, notset, 3.8)	no_status	❓
install-pkg (ubuntu-22.04, notset, 3.11)	no_status	❓
install-pkg (macOS-12, fabric, 3.8)	no_status	❓
install-pkg (macOS-12, fabric, 3.11)	no_status	❓
install-pkg (macOS-12, pytorch, 3.8)	no_status	❓
install-pkg (macOS-12, pytorch, 3.11)	no_status	❓
install-pkg (macOS-12, lightning, 3.8)	no_status	❓
install-pkg (macOS-12, lightning, 3.11)	no_status	❓
install-pkg (macOS-12, notset, 3.8)	no_status	❓
install-pkg (macOS-12, notset, 3.11)	no_status	❓
install-pkg (windows-2022, fabric, 3.8)	no_status	❓
install-pkg (windows-2022, fabric, 3.11)	no_status	❓
install-pkg (windows-2022, pytorch, 3.8)	no_status	❓
install-pkg (windows-2022, pytorch, 3.11)	no_status	❓
install-pkg (windows-2022, lightning, 3.8)	no_status	❓
install-pkg (windows-2022, lightning, 3.11)	no_status	❓
install-pkg (windows-2022, notset, 3.8)	no_status	❓
install-pkg (windows-2022, notset, 3.11)	no_status	❓

These checks are required after the changes to .github/workflows/ci-pkg-install.yml, src/lightning/app/__init__.py, requirements/app/app.txt.

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Thank you for your contribution! 💜

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