Error in the notebooks/Overview.ipynb notebook
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Do this:
``` python
squad_dataset = list_datasets(with_details=True)[datasets.index('squad')]
pprint(squad_dataset.__dict__)  # It's a simple python dataclass
``` Thanks! This worked. I have created a PR to fix this in the notebook. 
Hi,

I got the following error in **cell number 3** while exploring the **Overview.ipynb** notebook in google colab. I used the [link ](https://colab.research.google.com/github/huggingface/datasets/blob/master/notebooks/Overview.ipynb) provided in the main README file to open it in colab. 

```python
# You can access various attributes of the datasets before downloading them
squad_dataset = list_datasets()[datasets.index('squad')]

pprint(squad_dataset.__dict__)  # It's a simple python dataclass
```

Error message
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-8dc805c4949c> in <module>()
      2 squad_dataset = list_datasets()[datasets.index('squad')]
      3 
 ----> 4 pprint(squad_dataset.__dict__)  # It's a simple python dataclass
    
AttributeError: 'str' object has no attribute '__dict__'
```

The object `squad_dataset` is a `str` not a `dataclass` .
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