Not to set OMP_NUM_THREADS
Created by: wangkuiyi
When I am reading @reyoung 's tutorial on performance tuning, I learned that we'd want to make sure that OpenMP doesn't automatically start multiple threads by setting the environment variable OMP_NUM_THREADS=1 on the command line.
However, I also noticed that our Python source code will change the environment variable:
It seems that we should change the above code so that it doesn't change the value of the environment variable if it had been set.