The cfg is for creating different kinds of VGG nets(like vgg16 or vgg19). Feel free to dig into the source code of vgg.py located at
$YOUR_PYTHON3_DIR/lib/python3.6/site-packages/torchvision/models/vgg.py

It is very clear and simple how to use the cfg after you have a look at this file.
Plus, if you only want to extract feature, and not do any training stuff. @KrnTneja’s solution is better.