quantized elu: eager mode QAT handling (#40104)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40104
Adds eager mode QAT handling for quantized ELU.
Test Plan:
```
python test/test_quantization.py TestQuantizationAwareTraining.test_activations
```
Imported from OSS
Differential Revision: D22075082
fbshipit-source-id: 90eb06e4c52ec542fda97d7ee108a38465d3e845
