quantized elu: graph mode handling (#40111)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40111
Adds graph mode handling for quantized elu.
Test Plan:
```
python test/test_quantization.py TestQuantizeScriptPTSQOps.test_elu
```
Imported from OSS
Differential Revision: D22075080
fbshipit-source-id: 37fb1b9e390f2a33d47cbd025157532379b6aa64
