Fix qobserver test
Summary: Change the device config in qobserver test to a string to honor --device flag.
Test Plan: buck run caffe2/benchmarks/operator_benchmark/pt:qobserver_test -- --iterations 1 --device cpu
Reviewed By: ngimel
Differential Revision: D22536379
fbshipit-source-id: 8926b2393be1f52f9183f8205959a3ff18e3ed2a
