[op-bench] check device attribute in user inputs
Summary: The device attribute in the op benchmark can only include 'cpu' or 'cuda'. So adding a check in this diff.
Test Plan: buck run caffe2/benchmarks/operator_benchmark:benchmark_all_test -- --warmup_iterations 1 --iterations 1
Reviewed By: ngimel
Differential Revision: D22538252
fbshipit-source-id: 3e5af72221fc056b8d867321ad22e35a2557b8c3
