Created by: yihuaxu

Based on investigation of ERNIE, we found that layernorm is lack of multi-threads JIT implemention. This PR is to add the layernorm multi-threads optimizing by using OpenMP. But based on initial benchmark with ERNIE, single thread layernorm performance seems to be worse than before.

CPU Model: Intel(R) Xeon(R) Gold 6248 CPU @ 2.50GHz