[Feature Request] OpenMP support

[baixianger]

Motivation

Use openMP to parallel some code snippets. And is this necessary for envpool?

Solution

Add some macro to some parts of codes to parallel some for loop in envpool, like below:

#pragma omp xxxxx
for loop in c++

Reference

• [1] https://www.openmp.org/wp-content/uploads/OpenMPRefCard-5-2-web.pdf

Checklist

• I have checked that there is no similar issue in the repo (required)