Solved by Vali_Calinescu in post #2 
              
              
                Found the answer! 
The line 
self.features = nn.Sequential(*list(model_vgg.children())[:20]) 
should actually be 
self.features = nn.Sequential(*list(model_vgg.children())[0][:20]) 
I only wanted to extract the children form the first sequential layer of VGG-19 that has 3 layers(Sequential, Adaptive…