Fix iter_batches
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_The documentation is not available anymore as the PR was closed or merged._ I also ran the code in https://github.com/huggingface/datasets/issues/5111 and it works fine now :) This is ready for review :)
The `pa.Table.to_reader()` method available in `pyarrow>=8.0.0` may return chunks of size < `max_chunksize`, therefore `iter_batches` can return batches smaller than the `batch_size` specified by the user

Therefore batched `map` couldn't always use batches of the right size, e.g. this fails because it runs only on one batch of one element:
```python

from datasets import Dataset, concatenate_datasets

ds = concatenate_datasets([Dataset.from_dict({"a": [i]}) for i in range(10)])

ds2 = ds.map(lambda _: {}, batched=True)
assert list(ds2) == list(ds)
```

This was introduced in https://github.com/huggingface/datasets/pull/5030

Close https://github.com/huggingface/datasets/issues/5111

This will require a patch release along with https://github.com/huggingface/datasets/pull/5113

TODO:
- [x] fix tests
- [x] add more tests
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