Run userbenchmarks in subprocess in CI (#1584)
Summary:
This depends on https://github.com/pytorch/benchmark/issues/1580 and https://github.com/pytorch/benchmark/issues/1583!
Runs CI in subprocesses so it should be strictly safer!
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1584
Reviewed By: xuzhao9
Differential Revision: D45329729
Pulled By: janeyx99
fbshipit-source-id: 6a9919caa8cee6ad8b6b472df8c10d4ff3a0c582
