You can print the layers of your resnet using print(model).
The output shows you how the submodules are defined.
Since these blocks are not defined by a main nn.Sequential container, you would have to use the attribute names to index specific layers, e.g.:
model.layer4[1].conv3.register_forward_hook

This line would index the block layer4 (which is an nn.Sequential container), the second Bottleneck and the conv3 layer inside it.
Let me know, if this would work for you.