LiteDataLoader code improvements and docs

[awaelchli]

What does this PR do?

Part of #10297 (follow-up to #10279)
Related: #10329

Updates docs, fixes typing (a DataLoader is not an iterator).

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

cc @Borda @justusschock @awaelchli @akihironitta @ninginthecloud @carmocca

[tchaton]

LGTM ! This code was added for people not using a DataLoader

[awaelchli]

LGTM ! This code was added for people not using a DataLoader

In order to provide a seamless experience with all accelerators and strategies, we currently assume in both Lightning and Lite only DataLoaders.
If there is a desire to support non-DataLoader iterables, we will have to rethink a few components to make this work.

[justusschock]

@awaelchli

In order to provide a seamless experience with all accelerators and strategies, we currently assume in both Lightning and Lite only DataLoaders.

AFAIK this is not true for Lightning, where we explicitly have type checks against the data loader when replacing samplers etc. to make sure it also works with raw iterables (just not all features then)

[awaelchli]

AFAIK this is not true for Lightning, where we explicitly have type checks against the data loader when replacing samplers etc. to make sure it also works with raw iterables (just not all features then)

This claim has been made in the past but where do we see that in our tests, docs and examples? I am failing to create an example based on our boring model, as it will just throw an error.

[carmocca]

Thanks Adrian!

Would you mind also addressing the open reviews in #10279 ?