Bug fix: PDEOperatorCartesianProd (#1925)

Author: Jerry-Jzy

deepxde/data/pde_operator.py

@@ -307,7 +307,7 @@ def forward_call(trunk_input):
 
         losses_bc = zip(*losses_bc)
         losses_bc = [bkd.reduce_mean(bkd.stack(loss, 0)) for loss in losses_bc]
-        losses.append(losses_bc)
+        losses.extend(losses_bc)
         return losses
 
     def losses_train(self, targets, outputs, loss_fn, inputs, model, aux=None):