* New model.yaml `activation:` field

Add optional model yaml activation field to define model-wide activations, i.e.:

```yaml
activation: nn.LeakyReLU(0.1)  # activation with arguments
activation: nn.SiLU()  # activation with no arguments
```
Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Update yolo.py
Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>

* Add example models

* l to m models

* update

* Add yolov5s-LeakyReLU.yaml

* Update yolov5s-LeakyReLU.yaml
Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>
Signed-off-by: Glenn Jocher <glenn.jocher@ultralytics.com>