You need to pass value to the vgg_name variable used in the ‘init’ function.
Try like below:
class VGG(nn.Module):
def  init (self, cfg, vgg_name):
super(VGG, self). init ()
self.features = self._make_layers(cfg[vgg_name])
…
…
Then:
net = VGG(cfg, ‘VGG11’)