code: @Override
      public void backward() {
          modInX.getOutputAdj().elemAdd(yAdj);
          modInW.getOutputAdj().addValue(k, yAdj.getSum());
      }
docs: Backward pass:
  dG/dx_i += dG/dy_i dy_i/dx_i = dG/dy_i
  dG/dw_k += \sum_{i=1}^n dG/dy_i dy_i/dw_k = \sum_{i=1}^n dG/dy_i