Add cross-PR caching within CircleCI for gallery examples

[NicolasHug]

This PR adds cross-PR caching for the gallery examples. They're long to build ATM but this will be very much needed when we start building lengthy tutorials

According to my experiments on a custom branch it seems to work fine, see e.g. https://app.circleci.com/pipelines/github/pytorch/vision/8105/workflows/9b74c75a-d0c3-42c0-a812-90b3baf8df2e/jobs/579037 where we can read "after excluding 1 file that had previously been run (based on MD5)."

[fmassa]

This LGTM, but for my understanding, could you clarify a few things?

It looks like sphinx already avoids full recompilation if the gallery files were already present and nothing has changed, but if a change happens it recompiles only the changed files, is that correct?

[NicolasHug]

It looks like sphinx already avoids full recompilation if the gallery files were already present and nothing has changed, but if a change happens it recompiles only the changed files, is that correct?

Yes that's true, although for sphinx-gallery this is slightly more subtle, because SG generates .rst files that sphinx will consider as sources. In order not to re-generate files that didn't changed (so that sphinx doesn't consider those sources as "new"), SG keeps track of the md5 hashes of the examples in the auto_examples folder.

Locally everything works as expected, but in the CI it's different: the auto_examples folder isn't shared across PRs which means the examples are always re-computed. This PR is about caching within CircleCI.

[fmassa]

Thanks for the clarification!