produce diagram for caffe2 build matrix (#18517)
Summary:
This PR splits the configuration tree data from the logic used to construct the tree, for both `pytorch` and `caffe2` build configs.
Caffe2 configs are also now illustrated in a diagram.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18517
Differential Revision: D14936170
Pulled By: kostmo
fbshipit-source-id: 7b40a88512627377c5ea0f24765dabfef76ca279
