Solved by sbelharbi in post #5 
              
              
                if you are using pycharm + remote interpreter, try to clear its cache and delete all files of that env. 
also, make sure you dont have your own files named multiprocessing.py. 
here: 
conda create -n your_env python=3.7
conda install pytorch==1.9.0 torchvision==0.10.0 cudatoolkit=11.1 -c pytorch -c …