Operator support for `F.hardswish`

[shaharelys]

🚀 Feature

Implement HardSwish activation function.

Motivation

Relatively easy activation function implementation as a good first issue as nikitaved suggested under #64

Pitch

Add HardSwish (x * ReLU6(x + 3) / 6) leveraging existing ReLU6 support.

cc @apaz-cli