Add example of getting DataLoader from within LightningModule

[jgibson2]

What does this PR do?

Updates docs with example of getting the DataLoader object from within the LightningModule, motivated by #10834

Does your PR introduce any breaking changes? If yes, please list them.

No

Before submitting

• [n/a] Was this discussed/approved 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)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

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

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Did you have fun?

I did :)

[Borda]

@jgibson2 pls use the checklist in the PR 🐰

[jgibson2]

@jgibson2 pls use the checklist in the PR 🐰

Sorry, what does this mean?

[akihironitta]

@jgibson2 pls use the checklist in the PR rabbit

Sorry, what does this mean?

@jgibson2 He's just asking you to check the boxes in the description of this PR above :) but rohitgr7 took care of that already :)

[Borda]

please wrap all to classes and use .. testcode:: instead of .. code-block:: python

[tchaton]

LGTM !