quantized elu: require observation (#40100)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40100
ELU has a range of [-1, inf]. In the original PR which added
the quantized operator we decided to pass the quantization params
from the input. However, it makes more sense to require observation
for this op.
This PR changes the API to require observation. Next PRs in this stack
will add the eager and graph mode handling.
Test Plan:
```
python test/test_quantization.py TestQuantizedOps.test_qelu
```
Imported from OSS
Differential Revision: D22075083
fbshipit-source-id: 0ea0fd05a00cc7a5f122a2b1de09144bbd586f32
