Registers a child link to this chain.
Deprecated since version v2.0.0: Assign the child link directly to an attribute within
an initialization scope, instead.
For example, the following code
chain.add_link('l1', L.Linear(3, 5))
can be replaced by the following line.
chain.l1 = L.Linear(3, 5)
The latter one is easier for IDEs to keep track of the attribute’s