CircleCI configuration generator

One may no longer make changes to the .circleci/config.yml file directly. Instead, one must edit these Python scripts or files in the verbatim-sources/ directory.

Usage

1. Make changes to these scripts.
2. Run the regenerate.sh script in this directory and commit the script changes and the resulting change to config.yml.

You'll see a build failure on TravisCI if the scripts don't agree with the checked-in version.

Motivation

These scripts establish a single, authoritative source of documentation for the CircleCI configuration matrix. The documentation, in the form of diagrams, is automatically generated and cannot drift out of sync with the YAML content.

Furthermore, consistency is enforced within the YAML config itself, by using a single source of data to generate multiple parts of the file.

See pytorch#17038

Future direction

Declaring sparse config subsets

See comment here:

In contrast with a full recursive tree traversal of configuration dimensions,

in the future future I think we actually want to decrease our matrix somewhat and have only a few mostly-orthogonal builds that taste as many different features as possible on PRs, plus a more complete suite on every PR and maybe an almost full suite nightly/weekly (we don't have this yet). Specifying PR jobs in the future might be easier to read with an explicit list when we come to this.