[ONNX] Add `caffe2/python/onnx/**` to merge rule
This PR extends merge rule such that any related fixes needed for caffe2 onnx tests can be merged in the same PR.
Test skips need to be added to `caffe2/python/onnx/tests/onnx_backend_test.py` for new ONNX operators when ONNX submodule is updated.
