Issue: Load text file for RoBERTa pre-training. 

Comment:
Hi @lhoestq
Thanks for your suggestion.

I tried 
```
dataset = load_dataset('text', data_files='test.txt',cache_dir="./", split="train")
print(dataset)
dataset.set_format(type='torch',columns=["text"])
dataloader = torch.utils.data.DataLoader(dataset, batch_size=8)
next(iter(dataloader))
```

But it still doesn't work and got error:
```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-388aca337e2f> in <module>
----> 1 next(iter(dataloader))

/Library/Python/3.7/site-packages/torch/utils/data/dataloader.py in __next__(self)
    361 
    362     def __next__(self):
--> 363         data = self._next_data()
    364         self._num_yielded += 1
    365         if self._dataset_kind == _DatasetKind.Iterable and \

/Library/Python/3.7/site-packages/torch/utils/data/dataloader.py in _next_data(self)
    401     def _next_data(self):
    402         index = self._next_index()  # may raise StopIteration
--> 403         data = self._dataset_fetcher.fetch(index)  # may raise StopIteration
    404         if self._pin_memory:
    405             data = _utils.pin_memory.pin_memory(data)

/Library/Python/3.7/site-packages/torch/utils/data/_utils/fetch.py in fetch(self, possibly_batched_index)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

/Library/Python/3.7/site-packages/torch/utils/data/_utils/fetch.py in <listcomp>(.0)
     42     def fetch(self, possibly_batched_index):
     43         if self.auto_collation:
---> 44             data = [self.dataset[idx] for idx in possibly_batched_index]
     45         else:
     46             data = self.dataset[possibly_batched_index]

/Library/Python/3.7/site-packages/datasets-0.4.0-py3.7.egg/datasets/arrow_dataset.py in __getitem__(self, key)
   1069             format_columns=self._format_columns,
   1070             output_all_columns=self._output_all_columns,
-> 1071             format_kwargs=self._format_kwargs,
   1072         )
   1073 

/Library/Python/3.7/site-packages/datasets-0.4.0-py3.7.egg/datasets/arrow_dataset.py in _getitem(self, key, format_type, format_columns, output_all_columns, format_kwargs)
   1056                 format_columns=format_columns,
   1057                 output_all_columns=output_all_columns,
-> 1058                 format_kwargs=format_kwargs,
   1059             )
   1060         return outputs

/Library/Python/3.7/site-packages/datasets-0.4.0-py3.7.egg/datasets/arrow_dataset.py in _convert_outputs(self, outputs, format_type, format_columns, output_all_columns, format_kwargs)
    872                     continue
    873                 if format_columns is None or k in format_columns:
--> 874                     v = map_nested(command, v, **map_nested_kwargs)
    875                 output_dict[k] = v
    876         return output_dict

/Library/Python/3.7/site-packages/datasets-0.4.0-py3.7.egg/datasets/utils/py_utils.py in map_nested(function, data_struct, dict_only, map_list, map_tuple, map_numpy, num_proc, types)
    214     # Singleton
    215     if not isinstance(data_struct, dict) and not isinstance(data_struct, types):
--> 216         return function(data_struct)
    217 
    218     disable_tqdm = bool(logger.getEffectiveLevel() > INFO)

/Library/Python/3.7/site-packages/datasets-0.4.0-py3.7.egg/datasets/arrow_dataset.py in command(x)
    833                     if x.dtype == np.object:  # pytorch tensors cannot be instantied from an array of objects
    834                         return [map_nested(command, i, **map_nested_kwargs) for i in x]
--> 835                 return torch.tensor(x, **format_kwargs)
    836 
    837         elif format_type == "tensorflow":

TypeError: new(): invalid data type 'str'
```

I found type can be ['numpy', 'torch', 'tensorflow', 'pandas'] only, how can I deal with the string type?