MKLDNN elementwise_mul: Parallelize mul

paddle/fluid/operators/elementwise_mul_mkldnn_op.cc

@@ -144,6 +144,7 @@ class ElementwiseMulMKLDNNKernel : public framework::OpKernel<T> {
 
           mul_func_t mul_func = (mul_func_t) mul.getCode();
 
+          #pragma omp parallel for collapse(2)
           for (int ni = 0; ni < n; ni++) {
             for (int ci = 0; ci < C; ci++) {
               auto ptr_x =