code: @Override
      public void backward() {
          VarTensor tmp = new VarTensor(yAdj); // copy
          tmp.elemMultiply(y);
          tmp = tmp.copyAndConvertAlgebra(RealAlgebra.getInstance());
          VarTensor xAdj = modInX.getOutputAdj();
          xAdj.elemAdd(tmp);
      }
docs: Backward pass:
  dG/dx_i += dG/dy_i dy_i/dx_i = dG/dy_i exp(x_i)