Skip CPU benchmarks since we don't observe their stats (#1784)
Summary:
Save some CI time!
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1784
Reviewed By: xuzhao9
Differential Revision: D47737622
Pulled By: janeyx99
fbshipit-source-id: 97d8341010b28b67c8dca749a89ebe9310f5fa80