* Update cls bias init

Increased numerical precision. Returns 1.0 probability for single-class datasets now. Addresses https://github.com/ultralytics/yolov5/issues/5357

```python
torch.sigmoid(torch.tensor([math.log(0.6 / (1 - 0.99999))]))
Out[19]: tensor([1.0000])
```

* Update yolo.py