Move CI to GitHub Actions (#5045)

This PR migrates CircleCI workflow to GHA and makes a few minor changes: - Upload intermediary docker containers from GCR to ECR (as they are faster to download and do not incur egress costs to the infra) - For the same reason, workflows no longer require AWS credentials, as runners provision to have write access to the bucket/ECR - Skip the logic of rebasing PRs against latest trunk, as GH is doing it automatically - Build/publish docs on trunk in parallel to CPU tests build - Rely on GitHub reusable workflows and split workflow into `_build.yml`, `_test.yml` and `_docs.yml` Test plan: - Enable codecov collection on PRs and test that it works (see [this](https://github.com/pytorch/xla/actions/runs/5064520203/jobs/9092527785?pr=5045) log) - Test docs build (see [this](https://github.com/pytorch/xla/actions/runs/5071034648/jobs/9106921476) log)

Author: malfet