You could add your custom linear layers to model.fc as shown here:
model =  models.inception_v3()
fc_feat = model.fc.in_features
model.fc = nn.Sequential(
    nn.Linear(fc_feat, 512),
    nn.ReLU(),
    nn.Linear(512, 49*6),
    nn.Sigmoid(),
)

Let me know, if that would work for your use case.