Fix repo. fetch/checkout in PR slow CI job

[ydshieh]

What does this PR do?

Fix repo. fetch/checkout in PR slow CI job.

The previous way:

git fetch && git checkout ${{ github.event.pull_request.head.sha }}

only works for a pull request that is pushed to a branch of huggingface/transformers.

For PRs from outside contributors (so push to a forked repository), it will fail.

The new way

git fetch origin pull/${{ github.event.pull_request.number }}/head:pull/${{ github.event.pull_request.number }}/merge && git checkout pull/${{ github.event.pull_request.number }}/merge

works in both cases.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[amyeroberts]

Thanks!