[Hotfix] Remove reference to torch.jit.export

[SkafteNicki]

What does this PR do?

Fixes #<issue_number>

Pytorch 2.9 was release yesterday (https://github.com/pytorch/pytorch/releases/tag/v2.9.0) which means that our sphinx build which is pointing to the stable docs is now failing with

py:func reference target not found: torch.jit.export

since torch.jit.export has been removed. See docs html build on any PR right now, but here is an example:
https://github.com/Lightning-AI/pytorch-lightning/actions/runs/18559007926/job/52903249690?pr=21229

For now, restructure the problematic doc to not block all other PRs but we probably need to begin the process of removing torchscript support since it has been deprecated from torch. Created issue #21293 to track this.

Before submitting

• Was this discussed/agreed 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 list all the breaking changes introduced by this pull request?
• Did you update the CHANGELOG? (not for typos, docs, test updates, or minor internal changes/refactors)

PR review

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

Reviewer checklist

• Is this pull request ready for review? (if not, please submit in draft mode)
• Check that all items from Before submitting are resolved
• Make sure the title is self-explanatory and the description concisely explains the PR
• Add labels and milestones (and optionally projects) to the PR so it can be classified

---

📚 Documentation preview 📚: https://pytorch-lightning--21292.org.readthedocs.build/en/21292/