def zonotope_add(self, x: "torch.Tensor") -> "torch.Tensor":
        """
        Modification required compared to the normal torch conv layers.
        The bias is added only to the central zonotope term and not the error terms.

        :param x: zonotope input to have the bias added.
        :return: zonotope with the bias added to the central (first) term.
        """
        # unsqueeze to broadcast along height and width
        bias = torch.unsqueeze(self.bias, dim=-1)
        bias = torch.unsqueeze(bias, dim=-1)
        x[0] = x[0] + bias
        return x

        Modification required compared to the normal torch conv layers.
        The bias is added only to the central zonotope term and not the error terms.

        :param x: zonotope input to have the bias added.
        :return: zonotope with the bias added to the central (first) term.
        
art/estimators/certification/deep_z/deep_z.py