Such a good idea. 
So, we can define class Model() like:
class MyNewModel(nn.Module):
  def __init__(self):
    super(MyNewModel, self).__init__()
    self.pre_trained_model = pre_trained_model
    self.untrained_part = untrained_part
  def forward(self, x):
    x = self.pre_trained_model(x)
    x = self.untrained_part(x)

Thank you so much for your timely help.