I believe you can do something like this:
model = models.vgg16(pretrained=True)
#list all the modules in the model's classifier
mod = list(model_ft.classifier.children())
#Pop the last module and add 2 modules
mod.pop()
mod.append(nn.Linear(4096,100))
mod.append(nn.Linear(100,num_of_classes))

#Replace vgg16's classifier with this new classifier
new_classifier = nn.Sequential(*mod)
model.classifier = new_classifier