Compile guide for Fabric

[awaelchli]

What does this PR do?

Part of #18852

Adds a guide for speeding up models with torch.compile and advice in combination with Fabric.

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📚 Documentation preview 📚: https://pytorch-lightning--19330.org.readthedocs.build/en/19330/

cc @Borda @carmocca @justusschock @awaelchli

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[github-actions]

⚡ Required checks status: All passing 🟢

Groups summary

🟢 fabric: Docs

Check ID	Status
docs-make (fabric, doctest)	success	✅
docs-make (fabric, html)	success	✅

These checks are required after the changes to docs/source-fabric/advanced/compile.rst, docs/source-fabric/glossary/index.rst, docs/source-fabric/guide/index.rst, docs/source-fabric/index.rst, docs/source-fabric/levels/advanced.rst.

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Thank you for your contribution! 💜

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[carmocca]

TIL. I thought we were doing this by default already. What's blocking us?

[awaelchli]

Since we are usually quite sensitive with breaking changes, I wanted to make this experimental first for one release. I still need to do the work and run a batch of lit-gpt models with this settings to be sure this is not counter productive as a default. But happy to discuss this further.

[carmocca]

I don't expect any significant number of users to find and enable this private flag. If you are afraid because the release is close, that's fine. But I would enable it as soon as we can after so that at least nightly users try it out