How is our CI put together? What is the history of the CI? What constraints are under the CI? Why does the CI use Docker? Why are build and test split into two phases? Why are some parts of the CI so convoluted? How does the HUD work? What kinds of configurations is PyTorch tested under? How did we decide what configurations to test?  What are some of the weird CI configurations? What's up with the XLA CI? What's going on with the Facebook internal builds? 

Further reading.

• The CI HUD for viewing the status of master https://ezyang.github.io/pytorch-ci-hud/build/pytorch-master
• Structure of CI https://github.com/pytorch/pytorch/blob/master/.circleci/README.md
• How to debug Windows problems on CircleCI https://github.com/pytorch/pytorch/wiki/Debugging-Windows-with-Remote-Desktop-or-CDB-(CLI-windbg)-on-CircleCI