Support for decoding Image/Audio types in map when format type is not default one
_The documentation is not available anymore as the PR was closed or merged._ The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5252). All of your documentation changes will be reflected on that endpoint. Yes, if the image column is the first in the batch keys, it will decode the images because it reads the actual values. We could avoid this by checking the batch type, and if it's `LazyDict`, `num_examples` is equal to `len(batch.pa_table)`, which doesn't lead to decoding. Good idea. This can be done in a subsequent PR btw, since it's out of scope of the original goal of this PR Just fixed a small bug where it would show the pyarrow 10 warning about None -> empty lists conversions even with an Array2D with no nulls Fixed another bug when your map function returns a mix of LazyDict or regular dict and added some tests
Add support for decoding the `Image`/`Audio` types in `map` for the formats (Numpy, TF, Jax, PyTorch) other than the default one (Python).

Additional improvements:
* make `Dataset`'s "iter" API  cleaner by removing `_iter` and replacing `_iter_batches` with `iter(batch_size)` (also implemented for `IterableDataset`)
* iterate over arrow tables in `map` to avoid `_getitem` calls, which are much slower than `__iter__`/`iter(batch_size)`, when the `format_type` is not Python
* fix `_iter_batches` (now named `iter`) when `drop_last_batch=True` and `pyarrow<=8.0.0` is installed
* lazily extract and decode arrow data in the default format 

TODO:
* [x] update the `iter` benchmark in the docs (the `BeamBuilder` cannot load the preprocessed datasets from our bucket, so wait for this to be fixed (cc @lhoestq))

Fix https://github.com/huggingface/datasets/issues/3992, fix https://github.com/huggingface/datasets/issues/3756