code: def policy_backward(eph, epx, epdlogp, model):
      
      dW2 = np.dot(eph.T, epdlogp).ravel()
      dh = np.outer(epdlogp, model["W2"])
      # Backprop relu.
      dh[eph <= 0] = 0
      dW1 = np.dot(dh.T, epx)
      return {"W1": dW1, "W2": dW2}
docs: backward pass. (eph is array of intermediate hidden states)