Before revision:  However, how to find a suitable estimation for such hardness , then explicitly modeling adaptive computation depths are still not investigated.
After revision:  However, how to find a suitable estimation for such hardness , and thus improve efficiency. The main challenge is how to measure such hardness and decide the required depths (i.e., layers) to conduct. Previous works generally build a halting unit to decide whether the computation should continue or stop at each layer. As there is no specific supervision of depth selection, the halting unit may be under-optimized and inaccurate, which results in suboptimal and unstable performance when modeling sentences.