Give the option to write the metrics json elsewhere (#1580)
Summary:
This is a step towards running optim benchmarks in subprocesses by allowing the metrics files to be written to a tmp dir and then aggregated at the end.
Also changes -df to --df since -df can be confused with -d and -f combo.
THE ONLY FILE RELEVANT FOR REVIEW is userbenchmark/optim/__init__.py, everything else is from the prior dependency https://github.com/pytorch/benchmark/issues/1579
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1580
Reviewed By: xuzhao9
Differential Revision: D45328979
Pulled By: janeyx99
fbshipit-source-id: 8d9725f01825cdbe387a56745b95bcb80bd463a4
