RNN反向通过时间

Question

我正在调查RNN，在阅读本文后，了解了Backrpop-through-time如何在RNN上运行： https://arxiv.org/pdf/1610.02583.pdf

但我对以下实现（来自cs231）感到困惑：

 for t in reversed(xrange(T)):
        dh_current = dh[t] + dh_prev
        dx_t, dh_prev, dWx_t, dWh_t, db_t = rnn_step_backward(dh_current, cache[t])
        dx[t] += dx_t
        dh0 = dh_prev
        dWx += dWx_t
        dWh += dWh_t
        db += db_t

为什么要总结dh [t]和dh_prev渐变， dh_current = dh [t] + dh_prev ？

完整源代码：https://github.com/williamchan/cs231-assignment3/blob/master/cs231n/rnn_layers.py