Thanks for the reply.
Here is the result of print(layer_name, layer) inside the last loop:
0 Conv2d(256, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
1 ReLU(inplace)
2 Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
3 ReLU(inplace)
4 Conv2d(512, 512, kernel_size=(3, 3), stride=(1, 1), padding=(1, 1))
5 ReLU(inplace)

The 5 layers are listed (also shown in the list of keys of activations).