leaf variable --single-cls training bug fix (#593)

utils/utils.py

@@ -493,6 +493,7 @@ def compute_loss(p, targets, model):  # predictions, targets, model
     s = 3 / np  # output count scaling
     lbox *= h['giou'] * s
     lobj *= h['obj'] * s * (1.4 if np == 4 else 1.)
+    if model.nc > 1:
         lcls *= h['cls'] * s
     bs = tobj.shape[0]  # batch size