Support no pre-fetching

[carmocca]

What does this PR do?

Adds support for no-prefetching.
This was proposed by @justusschock in #11516 (comment).
Also means users do not need to patch the Loops to disable it as it is done by some ffcv users.

One drawback is that is_last_batch will not be correctly set in this case.

Part of #11538

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

None

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cc @Borda @justusschock @awaelchli @ninginthecloud

[tchaton]

Super super neat !