Update cls bias init (#5520)

* 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

Update cls bias init (#5520)
bfacfc6b · Glenn Jocher · GitHub · 36a4de18 · bfacfc6b
--- a/models/yolo.py
+++ b/models/yolo.py
@@ -201,7 +201,7 @@ class Model(nn.Module):
        for mi, s in zip(m.m, m.stride):  # from
            b = mi.bias.view(m.na, -1)  # conv.bias(255) to (3,85)
            b.data[:, 4] += math.log(8 / (640 / s) ** 2)  # obj (8 objects per 640 image)
-            b.data[:, 5:] += math.log(0.6 / (m.nc - 0.99)) if cf is None else torch.log(cf / cf.sum())  # cls
+            b.data[:, 5:] += math.log(0.6 / (m.nc - 0.999999)) if cf is None else torch.log(cf / cf.sum())  # cls
            mi.bias = torch.nn.Parameter(b.view(-1), requires_grad=True)

    def _print_biases(self):