Make introduction example run on devices

[awaelchli]

What does this PR do?

Fixes #18090

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cc @Borda

[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 pytorch_lightning: Docs

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

These checks are required after the changes to docs/source-pytorch/starter/introduction.rst.

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Thank you for your contribution! 💜

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