Since kwargs is passed to the VGG class, you can use it to specify e.g. num_classes:
model = models.vgg11(num_classes=11)

The kwargs argument is used in the current approach to use the specific models definitions, e.g. vgg11, vgg16, as an abstraction layer and pass additional arguments to the construction of the model.
If you want to be able to pass more arguments and think that changing the dropout probability is a missing feature, I would recommend to create a feature request on GitHub with your proposal.