graph mode: add hardswish op (#37524)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/37524
Adds graph mode handling for the hardswish op.
Test Plan:
```
python test/test_quantization.py TestQuantizeScriptPTSQOps.test_hardswish
```
Imported from OSS
Differential Revision: D21310365
fbshipit-source-id: 07e79943ed8095a5220be1582867b33597b85855
