Return Python float instead of numpy.float64 in sklearn metrics
{'login': 'lewtun', 'id': 26859204, 'node_id': 'MDQ6VXNlcjI2ODU5MjA0', 'avatar_url': 'https://avatars.githubusercontent.com/u/26859204?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lewtun', 'html_url': 'https://github.com/lewtun', 'followers_url': 'https://api.github.com/users/lewtun/followers', 'following_url': 'https://api.github.com/users/lewtun/following{/other_user}', 'gists_url': 'https://api.github.com/users/lewtun/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lewtun/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lewtun/subscriptions', 'organizations_url': 'https://api.github.com/users/lewtun/orgs', 'repos_url': 'https://api.github.com/users/lewtun/repos', 'events_url': 'https://api.github.com/users/lewtun/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lewtun/received_events', 'type': 'User', 'site_admin': False}
[]
[]
I opened an issue on the `sklearn` repo to understand why `numpy.float64` is the default: https://github.com/scikit-learn/scikit-learn/discussions/20490 It could be surprising at first to use `tolist()` on numpy scalars but it works ^^ did the same for Pearsonr here: https://github.com/huggingface/datasets/pull/2614
This PR converts the return type of all `sklearn` metrics to be Python `float` instead of `numpy.float64`.

The reason behind this is that our Hub evaluation framework relies on converting benchmark-specific metrics to YAML ([example](https://huggingface.co/datasets/autonlp/autonlp-benchmark-raft-neelalex__raft-test-neelalex__raft-predictions-3/blob/main/README.md#L11)) and the `numpy.float64` format produces garbage like:

```python
import yaml
from datasets import load_metric

metric = load_metric("accuracy")
score = metric.compute(predictions=[0,1], references=[0,1])
print(yaml.dump(score["accuracy"])) # output below
# !!python/object/apply:numpy.core.multiarray.scalar
# - !!python/object/apply:numpy.dtype
#   args:
#   - f8
#   - false
#   - true
#   state: !!python/tuple
#   - 3
#   - <
#   - null
#   - null
#   - null
#   - -1
#   - -1
#   - 0
# - !!binary |
#   AAAAAAAA8D8=
```
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/2612/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/2612', 'html_url': 'https://github.com/huggingface/datasets/pull/2612', 'diff_url': 'https://github.com/huggingface/datasets/pull/2612.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2612.patch', 'merged_at': '2021-07-09T13:03:54Z'}