Split Caffe2 CI into cmake-only and python builds (#15917)

Split Caffe2 CI into cmake-only and python builds (#15917)

Summary:
bypass-lint

- Change all Caffe2 builds to use setup.py instead of cmake
- Add a -cmake- Caffe2 build configuration that uses cmake and only builds cpp
- Move skipIfCI logic from onnx test scripts to the rest of CI logic
- Removal of old PYTHONPATH/LD_LIBRARY_PATH/etc. env management
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15917

Reviewed By: orionr

Differential Revision: D13637583

Pulled By: pjh5

fbshipit-source-id: c5c5639db0251ba12b6e4b51b2ac3b26a8953153