Enable auto-download for PAN-X / Wikiann domain in XTREME
{'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}
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{'url': 'https://api.github.com/repos/huggingface/datasets/pulls/2326', 'html_url': 'https://github.com/huggingface/datasets/pull/2326', 'diff_url': 'https://github.com/huggingface/datasets/pull/2326.diff', 'patch_url': 'https://github.com/huggingface/datasets/pull/2326.patch', 'merged_at': datetime.datetime(2021, 5, 7, 8, 41, 10)}
This PR replaces the manual download of the `PAN-X.lang` domains with an auto-download from a Dropbox link provided by the Wikiann author. We also add the relevant dummy data for these domains.

While re-generating `dataset_infos.json` I ran into a `KeyError` in the `udpos.Arabic` domain so have included a fix for this as well.
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