LiteDataLoader wrapper improvements

[awaelchli]

What does this PR do?

Follow up to #10279
Related: #10329

Adds small improvements around the init wrapper in Lite.

• Minimal docs
• Clearer variable names
• Separation of tests

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @justusschock @awaelchli @akihironitta @carmocca

[carmocca]

@awaelchli do you want to land this before we move this code to Lightning?

[tchaton]

LGTM !