add quantized ELU activation (#34267)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34267
Adds quantized ELU.
Test Plan:
```
python test/test_quantized.py TestQuantizedOps.test_qelu
```
still need to benchmark, saving that for after the review comments
Imported from OSS
Differential Revision: D20370953
fbshipit-source-id: fe941bf966f72dd9eee2c4b2ef45fe7afb50c866
