training fixes

NN/models/advanced_transformer_trading.py

@@ -1238,14 +1238,13 @@ class TradingTransformerTrainer:
                     break
             
             if needs_transfer:
-                # Move batch to device and DELETE original CPU tensors to prevent memory leak
+                # Move batch to device - iterate over copy of keys to avoid modification during iteration
                 batch_gpu = {}
-                for k, v in batch.items():
+                for k in list(batch.keys()):  # Create list copy to avoid modification during iteration
+                    v = batch[k]
                     if isinstance(v, torch.Tensor):
                         # Move to device (creates GPU copy)
                         batch_gpu[k] = v.to(self.device, non_blocking=True)
-                        # Delete CPU tensor immediately to free memory
-                        del batch[k]
                     else:
                         batch_gpu[k] = v