The following `multiprocessing` code should be moved into `if __name__ == '__main__':`

[Yonv1943]

To use multiprocessing, the following code should be moved into if __name__ == '__main__':

https://github.com/AI4Finance-Foundation/ElegantRL/blob/91c1c3d8747d5a3dcf83d4051ba15250b4a750a8/examples/tutorial_BipedalWalker-v3.py#L29-L48

.