The _hidden Sequential module will be created on the fly in your forward. Since you don’t assign it as a class attribute, this submodule won’t be stored.
What is your current use case? Would it work, if you initialize this module in your __init__ like the other ones?
If that’s not possible for whatever reason, you could still create the submodule in your first forward pass, assign it as a member, and check if it’s already created in the subsequent forward calls.