Batch and async training do not work with macOS/Windows and Python >= 3.8

[marioyc]

In Python 3.8 the default mode of multiprocessing for macOS was changed

For reference: https://github.com/chainer/chainerrl/issues/572

[elbamos]

I was able to workaround this by calling mp.set_start_method('fork') before main() in the example scripts.

[shreyassr123]

I am not able to run train_dqn_batch_ale.py. Further, mp.set_start_method('fork') is also not working for me. I am getting the following error : AttributeError: Can't pickle local object 'main..make_env'