fix quantized elu benchmark (#42318)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/42318
We forgot to update this benchmark when quantized elu's signature
changed to require observation, fixing.
Test Plan:
```
cd benchmarks/operator_benchmark
python -m pt.qactivation_test
```
Imported from OSS
Reviewed By: supriyar
Differential Revision: D22845251
fbshipit-source-id: 1443f6f0deac695715b1f2bd47f0f22b96dc72ca
