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