Run pytorch mobile benchmark in PEP (#28437)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28437
Add target to build speed_benchmark_torch for PEP.
Added a new argument `--report_pep` to print total runtime information for PEP. Can add per-op stats under this later.
Test Plan: https://our.intern.facebook.com/intern/aibench/details/664440309179004
Reviewed By: hl475
Differential Revision: D18062059
fbshipit-source-id: ca80e980ce8e48604782a15ac44dd8d403832817
