Your code will explicitly return the parameters only, which all should be leaves. You should check the is_leaf attribute of the parameters before and after concatenation manually.
a = nn.Parameter(torch.randn(1))
b = nn.Parameter(torch.randn(1))

print(a.is_leaf)
> True
print(b.is_leaf)
> True

c = torch.cat((a, b), dim=0)
print(c.is_leaf)
> False