Upgrading Transformers

[Smartappli]

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[iamtrask]

Hey @Smartappli! Thanks for the PR! Many on the team are likely to be off for the weekend, but i've flagged your PR for review (good chance upgrading the transformers package passes all the tests and goes swimmingly in the mean time — shouldn't be long).

Thanks for taking the time to submit a PR back to the codebase! Also if you're not already, feel free to join our slack (slack.openmined.org).