Collect CPU time using time_ns in run.py (#497)
Summary:
Collect CPU time using python 3.7's time_ns()
which has better precision than time(). Also
display the CPU time in milliseconds.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/497
Reviewed By: xuzhao9
Differential Revision: D31620125
Pulled By: aaronenyeshi
fbshipit-source-id: 0b44ceedb7a1f1702615ad61c3658a82208a7e5b
