The numbered indices in these modules names are created by the nn.Sequential module.
Layers without any parameters will still get the index, but won’t be shown in the state_dict.
Here is a small example:
model = nn.Sequential(
    nn.Linear(1, 1),
    nn.ReLU(),
    nn.Linear(1, 1),
    nn.ReLU(),
    nn.Sigmoid(),
    nn.Linear(1, 1))
print(model.state_dict().keys())
> odict_keys(['0.weight', '0.bias', '2.weight', '2.bias', '5.weight', '5.bias'])

As you can see, only the layer indices 0, 2, and 5 are shown, as these layers contain parameters.