add hardswish FP operator (#34747)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34747
Adds the hardswish FP operator from MobileNetV3 to PyTorch. This is for
common operator coverage, since this is widely used. A future PR will
add the quantized version. CUDA is saved for a future PR as well.
Test Plan:
tests pass:
```
python test/test_torch.py TestTorchDeviceTypeCPU.test_hardswish_cpu_float32
```
microbenchmark:
https://gist.github.com/vkuzo/b10d3b238f24e58c585314e8b5385aca
(batch_size == 1: 11.5GiB/s, batch_size == 4: 11.9GiB/s)
Imported from OSS
Differential Revision: D20451404
fbshipit-source-id: c7e13c9ab1a83e27a1ba18182947c82c896efae2
