TPU CI testing
Run TPU CI testing using CircleCI.

I sent a guide in Slack with steps needed on the owner side for CircleCI and Google Cloud to make this work (setting env vars, creating GKE cluster, and populating the dataset).


@LysandreJik I'm wondering if CircleCI does not run the pending changes to `.circleci/config.yml` if the changes came from a PR from a forked repo.

When testing on my private repo, I used a branch on the main repo and CircleCI did include the pending changes when running. I've seen lots of differences between branches on main repo and forked repo PRs when it comes to CircleCI and Github Actions.

The action item might be to remake this PR as a branch on the repo rather than a forked PR if you'd like to see the `job` run before submit.<|||||>


> @LysandreJik I'm wondering if CircleCI does not run the pending changes to `.circleci/config.yml` if the changes came from a PR from a forked repo.
> 
> When testing on my private repo, I used a branch on the main repo and CircleCI did include the pending changes when running. I've seen lots of differences between branches on main repo and forked repo PRs when it comes to CircleCI and Github Actions.
> 
> The action item might be to remake this PR as a branch on the repo rather than a forked PR if you'd like to see the `job` run before submit.

I think the forked repo is irrelevant. I was able to run new CircleCI changes in a similar PR for PyTorch Lightning: https://github.com/PyTorchLightning/pytorch-lightning/pull/2486
<|||||>Looks like this rebase went wrong - recreated the change in https://github.com/huggingface/transformers/pull/6158