[ONNX] Add `third_party/onnx` to merge rule (#84715)
We expect to bump onnx submodule version regularly to develop support for new onnx operators/functions. Adding this to merge rule reduces the burden for core maintainers for approval.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84715
Approved by: https://github.com/thiagocrepaldi, https://github.com/malfet
