Allowing dataloader prefetching to work with iterable datasets.

[Lucaweihs]

What does this PR do?

This PR fixes #34867 which noted that dataloader prefetching was not occurring for iterable datasets. This PR also refactors the dataloader creation code in the trainer so that the train/eval/test dataloaders all use the same underlying function rather than repeating the same logic in multiple places. I added two tests to confirm that the prefetching works with iterable datasets as expected.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• Did you write any new necessary tests?

Who can review?

@muellerzr @SunMarc