Fix allociné dataset configuration
Actually when there is only one configuration, then you don't need to specify the configuration in `load_dataset`. You can run:
```python
dataset = load_dataset('allocine')
```
and it works.

Maybe we should take that into account in the nlp viewer @srush ? @lhoestq Just to understand the exact semantics. Are you suggesting that if there is exactly 1 configuration I should not show the configuration menu and just treat it as if there were 0 configurations?  The configuration menu is fine imo.
It was more about the code snippet presented in the viewer.
For example for Allociné it currently shows this snippet to load the dataset:
```python
!pip install nlp
from nlp import load_dataset
dataset = load_dataset('allocine', 'allocine')
```
However for datasets with one or zero configurations, the second argument in `load_dataset` is optional. For Allociné, that has one configuration, we can expect to show instead:
```python
!pip install nlp
from nlp import load_dataset
dataset = load_dataset('allocine')
``` > Actually when there is only one configuration, then you don't need to specify the configuration in `load_dataset`. You can run:
> 
> ```python
> dataset = load_dataset('allocine')
> ```
> 
> and it works.
> 
> Maybe we should take that into account in the nlp viewer @srush ?

Oh ok, I didn't expect it would work! 

Anyway, I think it's intrinsically better to simply remove the optional parameter. 
The dummy data folder architecture seems also more logical this way.
 Fixed in the viewer. Checked that allocine works. Awesome thanks :)

Closing this.
This is a patch for #244. According to the [live nlp viewer](url), the Allociné dataset must be loaded with :
```python
dataset = load_dataset('allocine', 'allocine')
```
This is redundant, as there is only one "dataset configuration", and should only be:
```python
dataset = load_dataset('allocine')
```

This is my mistake, because the code for [`allocine.py`](https://github.com/huggingface/nlp/blob/master/datasets/allocine/allocine.py) was inspired by [`imdb.py`](https://github.com/huggingface/nlp/blob/master/datasets/imdb/imdb.py), which also force the user to specify the "dataset configuration" (even if there is only one).

I believe this PR should solve this issue, making the Allociné dataset more convenient to use.