[BUG] Metrics throwing new error on master since 0.4.0
{'login': 'jbragg', 'id': 2238344, 'node_id': 'MDQ6VXNlcjIyMzgzNDQ=', 'avatar_url': 'https://avatars.githubusercontent.com/u/2238344?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/jbragg', 'html_url': 'https://github.com/jbragg', 'followers_url': 'https://api.github.com/users/jbragg/followers', 'following_url': 'https://api.github.com/users/jbragg/following{/other_user}', 'gists_url': 'https://api.github.com/users/jbragg/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/jbragg/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/jbragg/subscriptions', 'organizations_url': 'https://api.github.com/users/jbragg/orgs', 'repos_url': 'https://api.github.com/users/jbragg/repos', 'events_url': 'https://api.github.com/users/jbragg/events{/privacy}', 'received_events_url': 'https://api.github.com/users/jbragg/received_events', 'type': 'User', 'site_admin': False}
[]
[]
Update - maybe this is only failing on bleu because I was not tokenizing inputs to the metric Closing - seems to be just forgetting to tokenize. And found the helpful discussion in huggingface/evaluate#105 
The following error occurs when passing in references of type `List[List[str]]` to metrics like bleu.
Wasn't happening on 0.4.0 but happening now on master.

```
  File "/usr/local/lib/python3.7/site-packages/nlp/metric.py", line 226, in compute
    self.add_batch(predictions=predictions, references=references)
  File "/usr/local/lib/python3.7/site-packages/nlp/metric.py", line 242, in add_batch
    batch = self.info.features.encode_batch(batch)
  File "/usr/local/lib/python3.7/site-packages/nlp/features.py", line 527, in encode_batch
    encoded_batch[key] = [encode_nested_example(self[key], cast_to_python_objects(obj)) for obj in column]
  File "/usr/local/lib/python3.7/site-packages/nlp/features.py", line 527, in <listcomp>
    encoded_batch[key] = [encode_nested_example(self[key], cast_to_python_objects(obj)) for obj in column]
  File "/usr/local/lib/python3.7/site-packages/nlp/features.py", line 456, in encode_nested_example
    raise ValueError("Got a string but expected a list instead: '{}'".format(obj))
```
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/519/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}