[typing] make mypy-protobuf output compatible with pyre for caffe2 type stubs (#55294)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/55294
Some static checkers like pyre have difficulties with types like `builtings.type`, so we strip the `builtins` prefix from autogened proto type stubs.
Test Plan: Let CI run.
Reviewed By: d4l3k
Differential Revision: D27477699
fbshipit-source-id: 45e19835974200a030817d37aec785e3ecb23e8b
