N.B.
N.B. Initialized to zero because this should *only* be used at the output layer, where zero initialization is appropriate
(Since version ) see corresponding Javadoc for more information.
Used at the output layer when we're only going to need some of the possible ouputs; it exposes the penultimate layer and then the Layer allows you to pass the results from that back in (caching it elsewhere) and only compute certain cells in the output layer (activationsFromPenultimateDot).