Function caffe2_imports::onnx_wrapper_get_empty_string_already_inited

pub fn onnx_wrapper_get_empty_string_already_inited<'a>() -> &'a String

| ONNX wrapper functions for protobuf’s | GetEmptyStringAlreadyInited() function used to | avoid duplicated global variable in the case | when protobuf is built with hidden visibility.