Clean the onnx constant fold code a bit (#19398)
Summary:
This is a follow up PR of https://github.com/pytorch/pytorch/pull/18698 to lint the code using clang-format.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19398
Differential Revision: D14994517
Pulled By: houseroad
fbshipit-source-id: 2ae9f93e66ce66892a1edc9543ea03932cd82bee
