Make numpy arrow extractor faster
{'login': 'lhoestq', 'id': 42851186, 'node_id': 'MDQ6VXNlcjQyODUxMTg2', 'avatar_url': 'https://avatars.githubusercontent.com/u/42851186?v=4', 'gravatar_id': '', 'url': 'https://api.github.com/users/lhoestq', 'html_url': 'https://github.com/lhoestq', 'followers_url': 'https://api.github.com/users/lhoestq/followers', 'following_url': 'https://api.github.com/users/lhoestq/following{/other_user}', 'gists_url': 'https://api.github.com/users/lhoestq/gists{/gist_id}', 'starred_url': 'https://api.github.com/users/lhoestq/starred{/owner}{/repo}', 'subscriptions_url': 'https://api.github.com/users/lhoestq/subscriptions', 'organizations_url': 'https://api.github.com/users/lhoestq/orgs', 'repos_url': 'https://api.github.com/users/lhoestq/repos', 'events_url': 'https://api.github.com/users/lhoestq/events{/privacy}', 'received_events_url': 'https://api.github.com/users/lhoestq/received_events', 'type': 'User', 'site_admin': False}
[]
[]
Looks like we have a nice speed up in some benchmarks. For example:
- `read_formatted numpy 5000`:  4.584777 sec -> 0.487113 sec
- `read_formatted torch 5000`:  4.565676 sec -> 1.289514 sec Can we convert this draft to PR @lhoestq ? Ready for review ! cc @vblagoje @lhoestq I tried the branch and it works for me. Although performance trace now shows a speedup, the overall pre-training speed up is minimal. But that's on my plate to explore further.  Thanks for investigating @vblagoje 

@albertvillanova , do you have any comments on this PR ? Otherwise I think we can merge it
I changed the NumpyArrowExtractor to call directly to_numpy and see if it can lead to speed-ups as discussed in https://github.com/huggingface/datasets/issues/2498

This could make the numpy/torch/tf/jax formatting faster
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/2505/reactions', 'total_count': 2, '+1': 0, '-1': 0, 'laugh': 0, 'hooray': 0, 'confused': 0, 'heart': 2, 'rocket': 0, 'eyes': 0}
{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2505', 'html_url': 'https://github.com/huggingface/datasets/pull/2505', 'diff_url': 'https://github.com/huggingface/datasets/pull/2505.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2505.patch', 'merged_at': '2021-06-28T09:53:38Z'}