That’s because vgg19 doesn’t have a fc member variable. Instead, it has a
  (classifier): Sequential (
    (0): Dropout (p = 0.5)
    (1): Linear (25088 -> 4096)
    (2): ReLU (inplace)
    (3): Dropout (p = 0.5)
    (4): Linear (4096 -> 4096)
    (5): ReLU (inplace)
    (6): Linear (4096 -> 100)
  )

To replace the last linear layer, a temporary solution would be
vgg19.classifier._modules['6'] = nn.Linear(4096, 8)