Add operator benchmark for 4bit/8bit embedding lookups
Summary: Add operator benchmark for 4bit/8bit embedding lookups in `aibench`.
Test Plan:
```
buck build //caffe2/benchmarks/operator_benchmark/pt:qembedding_bag_lookups_test
aibench-cli adhoc -c 'buck run //caffe2/benchmarks/operator_benchmark/pt:qembedding_bag_lookups_test'
````
The run was successful in aibench: https://www.internalfb.com/intern/aibench/details/738300474
https://www.internalfb.com/intern/aibench/details/346463246
Reviewed By: radkris-git
Differential Revision: D24268413
fbshipit-source-id: 7fb4ff75da47f8f327edab562c5d29bb69e00b8d
