check for fx2trt only for cuda device (#1395)
Summary:
this change is required to make the cpu device (-d cpu) work on the latest commit.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1395
Reviewed By: davidberard98
Differential Revision: D42996480
Pulled By: xuzhao9
fbshipit-source-id: a2c26cdc48096cc605a5f3bc010dbbf3911ab655
