Unwanted progress bars when accessing examples
doing a patch release now :)
When accessing examples from a dataset formatted for pytorch, some progress bars appear when accessing examples:
```python
In [1]: import datasets as ds                                        

In [2]: d = ds.Dataset.from_dict({"a": [0, 1, 2]}).with_format("torch")                                                           

In [3]: d[0]                                                         
100%|████████████████████████████████| 1/1 [00:00<00:00, 3172.70it/s]
Out[3]: {'a': tensor(0)}
```

This is because the pytorch formatter calls `map_nested` that uses progress bars

cc @sgugger 