Hi. The context needs to be set at the very beginning of the file you are executing. And you also need to execute your file from a terminal. From my experience, setting a spawn contex on a jupyter notebook won’t work. So your file needs to begin with something like that 
import torch.multiprocessing as mp
if __name__  ==  '__main__' :
    mp.set_start_method("spawn")
#do other imports, etc