The "all" split breaks streaming
Thanks for reporting @cakiki.

Yes, this is a bug. We are investigating it. @albertvillanova Nice! Let me know if it's something I can fix my self; would love to contribtue! @cakiki I was working on this but if you would like to contribute, go ahead. I will close my PR. ;)

For the moment I just pushed the test (to see if it impacts other tests). It impacted the test `test_generator_based_download_and_prepare` and I have fixed this.

So that you can copy the test I implemented in my PR and then implement a fix for this issue that passes the test `tests/test_builder.py::test_builder_as_streaming_dataset`. Hi @cakiki are you still interested in working on this? Are you planning to open a PR? Hi @albertvillanova ! Sorry it took so long; I wanted to spend this weekend working on it.
## Describe the bug

Not sure if this is a bug or just the way streaming works, but setting `streaming=True` did not work when setting `split="all"`

## Steps to reproduce the bug
The following works:
```python
ds = load_dataset('super_glue', 'wsc.fixed', split='all')
```
The following throws `ValueError: Bad split: all. Available splits: ['train', 'validation', 'test']`:

```python
ds = load_dataset('super_glue', 'wsc.fixed', split='all', streaming=True)
```

## Expected results
An iterator over all splits.

## Actual results
I had to do the following to achieve the desired result:
```python
from itertools import chain
ds = load_dataset('super_glue', 'wsc.fixed', streaming=True)
it =  chain.from_iterable(ds.values())
```

## Environment info
<!-- You can run the command `datasets-cli env` and copy-and-paste its output below. -->
- `datasets` version: 2.3.2
- Platform: Linux-4.15.0-176-generic-x86_64-with-glibc2.31
- Python version: 3.10.5
- PyArrow version: 8.0.0
- Pandas version: 1.4.3
