DDP `nl` fix (#5332)

train.py

@@ -246,7 +246,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
         model = DDP(model, device_ids=[LOCAL_RANK], output_device=LOCAL_RANK)
 
     # Model parameters
-    nl = model.model[-1].nl  # number of detection layers (to scale hyps)
+    nl = de_parallel(model).model[-1].nl  # number of detection layers (to scale hyps)
     hyp['box'] *= 3. / nl  # scale to layers
     hyp['cls'] *= nc / 80. * 3. / nl  # scale to classes and layers
     hyp['obj'] *= (imgsz / 640) ** 2 * 3. / nl  # scale to image size and layers