fix(nbeats): fix loss for backcast part

src/backends/torch/native/templates/nbeats.h

@@ -366,17 +366,19 @@ namespace dd
                           _backcast_length + _forecast_length);
     }
 
-    virtual torch::Tensor loss(std::string loss, torch::Tensor input,
+    virtual torch::Tensor loss(std::string loss, torch::Tensor input_real,
                                torch::Tensor output, torch::Tensor target)
     {
       torch::Tensor x_pred = torch::slice(output, 1, 0, _backcast_length);
       torch::Tensor y_pred = torch::slice(output, 1, _backcast_length,
                                           _backcast_length + _forecast_length);
+      torch::Tensor input_zeros = torch::zeros_like(input_real);
       if (loss.empty() || loss == "L1" || loss == "l1")
-        return torch::l1_loss(y_pred, target) + torch::l1_loss(x_pred, input);
+        return torch::l1_loss(y_pred, target)
+               + torch::l1_loss(x_pred, input_zeros);
       if (loss == "L2" || loss == "l2" || loss == "eucl")
         return torch::mse_loss(y_pred, target)
-               + torch::mse_loss(x_pred, input);
+               + torch::mse_loss(x_pred, input_zeros);
       throw MLLibBadParamException("unknown loss " + loss);
     }