Add executor and fuser options to the fastrnn test fixture (#42946)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42946
There are 3 options for the executor and fuser and some of them aren't
super interesting so I've combined the options into a single parameter, but
made it fairly easy to expand the set if there are other configs we might care
about.
Test Plan:
Benchmark it
Imported from OSS
Reviewed By: zheng-xq
Differential Revision: D23090177
fbshipit-source-id: bd93a93c3fc64e5a4a847d1ce7f42ce0600a586e
