Is this is a reason that I get None for the last lines of code?

Yes. All the leaf Tensors will have None as a grad_fn. This is expected.

Why does this happen?

Because .parameter() returns a python iterator. So you can only iterate through it once. If you want to be able to go through it multiple times, you can use the iterator to create a list to be able to go through the list later one: large_lr_layers = list(large_lr_layers).