[Manual data dir] Error message: nlp.load_dataset('xsum') -> TypeError
v 0.1.0 from pip

```python
import nlp
xsum = nlp.load_dataset('xsum')
```

Issue is `dl_manager.manual_dir`is `None`

```python

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-42-8a32f066f3bd> in <module>
----> 1 xsum = nlp.load_dataset('xsum')

~/miniconda3/envs/nb/lib/python3.7/site-packages/nlp/load.py in load_dataset(path, name, version, data_dir, data_files, split, cache_dir, download_config, download_mode, ignore_verifications, save_infos, **config_kwargs)
    515         download_mode=download_mode,
    516         ignore_verifications=ignore_verifications,
--> 517         save_infos=save_infos,
    518     )
    519 

~/miniconda3/envs/nb/lib/python3.7/site-packages/nlp/builder.py in download_and_prepare(self, download_config, download_mode, ignore_verifications, save_infos, dl_manager, **download_and_prepare_kwargs)
    361                 verify_infos = not save_infos and not ignore_verifications
    362                 self._download_and_prepare(
--> 363                     dl_manager=dl_manager, verify_infos=verify_infos, **download_and_prepare_kwargs
    364                 )
    365                 # Sync info

~/miniconda3/envs/nb/lib/python3.7/site-packages/nlp/builder.py in _download_and_prepare(self, dl_manager, verify_infos, **prepare_split_kwargs)
    397         split_dict = SplitDict(dataset_name=self.name)
    398         split_generators_kwargs = self._make_split_generators_kwargs(prepare_split_kwargs)
--> 399         split_generators = self._split_generators(dl_manager, **split_generators_kwargs)
    400         # Checksums verification
    401         if verify_infos:

~/miniconda3/envs/nb/lib/python3.7/site-packages/nlp/datasets/xsum/5c5fca23aaaa469b7a1c6f095cf12f90d7ab99bcc0d86f689a74fd62634a1472/xsum.py in _split_generators(self, dl_manager)
    102         with open(dl_path, "r") as json_file:
    103             split_ids = json.load(json_file)
--> 104         downloaded_path = os.path.join(dl_manager.manual_dir, "xsum-extracts-from-downloads")
    105         return [
    106             nlp.SplitGenerator(

~/miniconda3/envs/nb/lib/python3.7/posixpath.py in join(a, *p)
     78     will be discarded.  An empty last part will result in a path that
     79     ends with a separator."""
---> 80     a = os.fspath(a)
     81     sep = _get_sep(a)
     82     path = a

TypeError: expected str, bytes or os.PathLike object, not NoneType


```
