Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] Docs change / refactoring / dependency upgrade

Motivation and Context / Related issue

Since Torch 2.0, there is now a cleaner way of accessing the training dataloader in the LightningModule.configure_optimizers function.

https://github.com/Lightning-AI/pytorch-lightning/issues/10430#issuecomment-1487753339

This PR applies that change to the Lightning MNIST example notebook.

How Has This Been Tested (if it applies)

Checklist

[x] The documentation is up-to-date with the changes I made.
[x] I have read the CONTRIBUTING document and completed the CLA (see CONTRIBUTING).
[x] All tests passed, and additional code has been covered with new tests.

Hi @eonu!

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pytorch / opacus