Dear @smth thank you for your reply. However, output_cat  does not contain anything . Because my operation have inside unsqueeze, then error is thrown cannot unsqueeze empty tensor . I addtion, I don’t see the connection between both output_cat and output.append.  Also, if i try to replace output.append with output_cat.append, the error says that  '‘NoneType’ object has no attribute ‘append’ .
Thank you.