Fix iterable interface expected by numpy
It's been fixed in this commit: https://github.com/huggingface/datasets/commit/549110e08238b3716a5904667095fb003acda54e

Basically #2246 broke querying an index with a simple iterable.
With the fix, it's again possible to use iterables and we can keep RandIter as it is.

Closing since the fix is already on master
Numpy expects the old iterable interface with `__getitem__` instead of `__iter__`.