Use multiprocess from pathos for multiprocessing
{'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}
[]
[]
We can just install multiprocess actually, I'll change that Just an FYI: I remember that I wanted to try pathos a couple of years back and I ran into issues considering cross-platform; the code would just break on Windows. If I can verify this PR by running CPU tests on Windows, let me know! That's good to know thanks
I guess we can just wait for #644 to be merged first. I'm working on fixing the tests for windows Looks like all the CI jobs on windows passed !
I also tested locally on my windows and it works great :) 

I think this is ready to merge, let me know if you have any remarks @thomwolf @BramVanroy 
[Multiprocess](https://github.com/uqfoundation/multiprocess) (from the [pathos](https://github.com/uqfoundation/pathos) project) allows to use lambda functions in multiprocessed map.
It was suggested to use it by @kandorm.

We're already using dill which is its only dependency.
{'url': 'https://api.github.com/repos/huggingface/datasets/issues/656/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/656', 'html_url': 'https://github.com/huggingface/datasets/pull/656', 'diff_url': 'https://github.com/huggingface/datasets/pull/656.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/656.patch', 'merged_at': '2020-09-28T14:45:39Z'}