[onnxifi] Get rid of class member (#49380)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/49380
Couldn't resist removing a class member that is only used in one function.
Reviewed By: yinghai
Differential Revision: D25547366
fbshipit-source-id: 74e61c6a0068566fb7956380862999163e7e94bf
