add quantized_hardtanh (#34097)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34097
Adds quantized hardtanh. Calls the clamp kernel behind the
scenes.
Test Plan:
```
python test/test_quantized.py
```
Imported from OSS
Differential Revision: D20208860
fbshipit-source-id: 165a6a1c22f1dcc479679e5ea0c990d0e9c3b6c5
