Fix methods using `IterableDataset.map` that lead to `features=None`
State: closed | Author: alvarobartt (MEMBER)

As currently `IterableDataset.map` is setting the `info.features` to `None` every time as we don't know the output of the dataset in advance, `IterableDataset` methods such as `rename_column`, `rename_columns`, and `remove_columns`. that internally use `map` lead to the features being `None`.

This PR is related to #3888, #5245, and #5284 

## ✅ Current solution

The code in this PR is basically making sure that if the features were there since the beginning and a `rename_column`/`rename_columns` happens, those are kept and the rename is applied to the `Features` too. Also, if the features were not there before applying `rename_column`, `rename_columns` or `remove_columns`, a batch is prefetched and the features are being inferred (that could potentially be part of `IterableDataset.__init__` in case the `info.features` value is `None`).

## 💡 Ideas

Some ideas were proposed in https://github.com/huggingface/datasets/issues/3888, but probably the most consistent solution even though it may take some time is to actually do the type inferencing during the `IterableDataset.__init__` in case the provided `info.features` is `None`, otherwise, we can just use the provided features.

Additionally, as mentioned at https://github.com/huggingface/datasets/issues/3888, we could also include a `features` parameter to the `map` function, but that's probably more tedious.

Also thanks to @lhoestq for sharing some ideas in both https://github.com/huggingface/datasets/issues/3888 and https://github.com/huggingface/datasets/issues/5245 :hugs:

Comments: _The documentation is not available anymore as the PR was closed or merged._

_The documentation is not available anymore as the PR was closed or merged._

Maybe other options are:
* Keep the `info.features` to `None` if those were initially `None`
* Infer the features with pre-fetching just if the `info.features` is `None`
* If the `info.features` are there, make sure that after `map` features is not `None`

Hi @lhoestq something that's still not clear to me is: should we infer the features always when applying a `map` if those are initially `None`, or just assume that if the features are initially `None` those should be left that way unless the user specifically sets those (or during iter)?

In this PR I'm using `from datasets.iterable_dataset import _infer_features_from_batch` to infer the features when those are `None` using pre-fetch of `self._head()`, but I'm not sure if that's the expected behavior.

Thanks in advance for your help!

Also, the PR still has some more work to do, but probably the most relevant thing to fix right now is that the `features` are being set to `None` in the functions `IterableDataset.rename_column`, `IterableDataset.rename_columns`, and `IterableDataset.remove_columns` when the `features` originally had a value. So once that's fixed maybe we can focus on improving the current `map`'s behavior, so as to avoid this from happening also when the user uses `map` directly and not through the functions mentioned above.

> Cool thank you ! Resolving the features can be expensive sometimes, so maybe we don't resolve the features and we can just rename/remove columns if the features are known (i.e. if they're not None). What do you think ?

Thanks for the feedback! Makes sense to me 👍🏻 I'll commit the comments now!

Already done @lhoestq, feel free to merge whenever you want! Also before merging, can you please link the following issues https://github.com/huggingface/datasets/issues/3888, https://github.com/huggingface/datasets/issues/5245, and https://github.com/huggingface/datasets/issues/5284, so that those are closed upon merge? Thanks!