Fix key error in webdataset

I was running into

```
example[field_name] = {"path": example["__key__"] + "." + field_name, "bytes": example[field_name]}
KeyError: 'png'
```

The issue is that a filename may have multiple "." e.g. `22.05.png`. Changing `split` to `rsplit` fixes it.

Related https://github.com/huggingface/datasets/issues/6880

Comments:
hi ! What version of `datasets` are you using ? Is this issue also happening with `datasets==3.0.0` ?
Asking because we made sure to replicate the official webdataset logic, which is to use the latest dot as separator between the sample base name and the key

Hi, yes this is still a problem on `datasets==3.0.0`.

I was using `datasets=2.20.0` and in that version you get the key error.

I just upgraded to `datasets==3.0.0` and in this version, you do not get a key error because it sets all keys to none by default   in `_generate_examples` function:

```python
if field_name not in example:
    example[field_name] = None
```

However, the behaviour is still incorrect. This `if` condition is triggered because the filename is not split properly and it returns the data as `None` when it shouldn't.

> we made sure to replicate the official webdataset logic, which is to use the latest dot as separator
 
Ah, but that's not what `split(, 1)` does though. This is exactly why I'm suggesting to use `rsplit` instead. In general, using `rsplit` should not be a breaking change I believe.

Hi  @ragavsachdeva,

We already had this discussion in the issue you have linked:
- #6880
- I even opened a PR with your proposed fix:
  - #6888

However, we decided not to implement this feature because it is NOT aligned with the behavior of the `webdataset` library:
> The prefix of a file is all directory components of the file plus the file name component up to the *first* “.” in the file name.
```python
In [1]: import webdataset as wds

In [2]: wds.tariterators.base_plus_ext("22.05.png")
Out[2]: ('22', '05.png')
```



Ah, my apologies I missed https://github.com/huggingface/datasets/pull/6888 (clearly didn't do my due diligence). It's such a weird convention to have though. My keys are `/some/path/22.0/1.1.png` and it splits them at  `/some/path/22` and `.0/1.1.png`(!) I'm okay with this PR not being merged though. Thanks for your time.

Actually `datasets` is not behaving correctly in this case and should not split as `.0/1.1.png` - even webdataset handles this correctly via their regex `^((?:.*/|)[^.]+)[.]([^/]*)$` in `wds.tariterators.base_plus_ext` here:

https://github.com/webdataset/webdataset/blob/87bd5aa41602d57f070f65a670893ee625702f2f/webdataset/tariterators.py#L36

Oh.. the intention with that regex is to capture "multi-part" extensions e.g. `.tar.gz`. Makes sense. So `rsplit` isn't the solution then and neither is `split`. This expression makes so much more sense. Nice find! I'm assuming you'll add a patch?

Issue addressed by:
- #7151

 Maybe the libray should suggest/warn users about **file naming**, in particular, file naming **with dots**. Indeed, otherwise a naive naming approach like mentioned above would lead to

```python
ValueError: The TAR archives of the dataset should be in WebDataset format, but the files in the archive don't share the same prefix or the same types.
```

with the following code

```ipython
In [1]: from datasets import load_dataset

In [2]: !tar tvf easy_trouble/001.tar
drwxr-xr-x phunc20/phunc20   0 2025-01-15 17:57 001/
-rw-r--r-- phunc20/phunc20  20 2025-01-15 17:54 001/1.2.jpg
-rw-r--r-- phunc20/phunc20  20 2025-01-15 17:53 001/1.1.jpg
-rw-r--r-- phunc20/phunc20  20 2025-01-15 17:57 001/2.1.jpg
-rw-r--r-- phunc20/phunc20  20 2025-01-15 17:57 001/2.2.jpg
-rw-r--r-- phunc20/phunc20  20 2025-01-15 17:57 001/2.3.jpg

In [29]: dataset = load_dataset("webdataset", data_dir="easy_trouble")
```

BTW, @ragavsachdeva , why would you find including a `.tar.gz` inside a `.tar` makes sense? I personally find switching `base_plus_ext` into sth similar to the following makes more sense, though. (That is, no multi-part extension.)

```python
from pathlib import Path

def base_plus_ext(path):
    """Split off all file extensions.

    Returns base, allext.
    """
    p = Path(path)
    base = (p.parent / p.stem).as_posix()
    ext = p.suffix.split(".")[-1]
    return base, ext
```