Update metadata if dataset features are modified
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["@lhoestq I'll try to add a test later if you think this approach with the wrapper is good."
 'Awesome thank you !\r\nYes this approach with a wrapper is good :)'
 "@lhoestq Added a test. To verify that this change fixes the problem, replace:\r\n```\r\n!pip install datasets==1.5\r\n```\r\nwith:\r\n```\r\n!pip install git+https://github.com/mariosasko/datasets-1.git@update-metadata\r\n```\r\nin the first cell of the notebook that is attached to the linked issue.\r\n\r\nThe CI failure is unrelated I think (building the docs locally doesn't throw an error)."
 'The CI fail for the docs has been fixed on master.\r\nMerging :)']
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This PR adds a decorator that updates the dataset metadata if a previously executed transform modifies its features. 
Fixes #2083 

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