[Metal] Add hardswish and hardsigmoid to metal, fix broadcasting for binary elementwise ops
Summary:
Add hardswish_ and hardsigmoid_ activations to enable MobileNetV3.
Also fix binary elementwise ops to work when the first input is being broadcasted rather than the second.
Test Plan:
Test on device:
```
arc focus2 pp-ios
```
Test on mac
```
buck test pp-macos
```
Reviewed By: xta0
Differential Revision: D26241385
fbshipit-source-id: 6ce7269d60d63cf909b75a7f4e18fb17ac2f5d31
