[PyTorch] Fix operator perf observer index issue.
Summary: Fix operator perf observer index issue.
Test Plan:
make sure that the operator index is populated correctly, ran benchmarking for pytext_mobile_inference, see result:
https://www.internalfb.com/intern/aibench/details/598900068317693
Reviewed By: linbinyu
Differential Revision: D21779222
fbshipit-source-id: 0fc3561d83d10cfabd73e1e6b6ee240ce0bafd80
