Fix automatic optimizer LR with DDP training (#18842)

ultralytics/engine/trainer.py

@@ -271,7 +271,6 @@ class BaseTrainer:
         )
         if world_size > 1:
             self.model = nn.parallel.DistributedDataParallel(self.model, device_ids=[RANK], find_unused_parameters=True)
-            self.set_model_attributes()  # set again after DDP wrapper
 
         # Check imgsz
         gs = max(int(self.model.stride.max() if hasattr(self.model, "stride") else 32), 32)  # grid size (max stride)
@@ -782,7 +781,7 @@ class BaseTrainer:
                 f"ignoring 'lr0={self.args.lr0}' and 'momentum={self.args.momentum}' and "
                 f"determining best 'optimizer', 'lr0' and 'momentum' automatically... "
             )
-            nc = getattr(model, "nc", 10)  # number of classes
+            nc = self.data.get("nc", 10)  # number of classes
             lr_fit = round(0.002 * 5 / (4 + nc), 6)  # lr0 fit equation to 6 decimal places
             name, lr, momentum = ("SGD", 0.01, 0.9) if iterations > 10000 else ("AdamW", lr_fit, 0.9)
             self.args.warmup_bias_lr = 0.0  # no higher than 0.01 for Adam