Simplify lightning example dataloader access

[eonu]

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)
• 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.

Lightning-AI/pytorch-lightning#10430 (comment)

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

How Has This Been Tested (if it applies)

Checklist

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

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