Fix seqeval metric
{'login': 'sgugger', 'id': 35901082, 'node_id': 'MDQ6VXNlcjM1OTAxMDgy', 'avatar_url': 'https://avatars.githubusercontent.com/u/35901082?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/sgugger', 'html_url': 'https://github.com/sgugger', 'followers_url': 'https://api.github.com/users/sgugger/followers', 'following_url': 'https://api.github.com/users/sgugger/following{/other_user}', 'gists_url': 'https://api.github.com/users/sgugger/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/sgugger/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/sgugger/subscriptions', 'organizations_url': 'https://api.github.com/users/sgugger/orgs', 'repos_url': 'https://api.github.com/users/sgugger/repos', 'events_url': 'https://api.github.com/users/sgugger/events{/privacy}', 'received_events_url': 'https://api.github.com/users/sgugger/received_events', 'type': 'User', 'site_admin': False}
[]
[]
The current seqeval metric returns the following error when computed:
```
~/.cache/huggingface/modules/datasets_modules/metrics/seqeval/78a944d83252b5a16c9a2e49f057f4c6e02f18cc03349257025a8c9aea6524d8/seqeval.py in _compute(self, predictions, references, suffix)
    102         scores = {}
    103         for type_name, score in report.items():
--> 104             scores[type_name]["precision"] = score["precision"]
    105             scores[type_name]["recall"] = score["recall"]
    106             scores[type_name]["f1"] = score["f1-score"]

KeyError: 'LOC'
```
This is because the current code basically tries to do:
```
scores = {}
scores["LOC"]["precision"] = some_value
```
which does not work in python. This PR fixes that while keeping the previous nested structure of results, with the same keys.
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/810/reactions', 'total_count': 0, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 0, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/810', 'html_url': 'https://github.com/huggingface/datasets/pull/810', 'diff_url': 'https://github.com/huggingface/datasets/pull/810.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/810.patch', 'merged_at': '2020-11-09T14:04:27Z'}