Nodes of different colours represent the following:
Solid arrows point from a procedure to one which it calls. Dashed
arrows point from an interface to procedures which implement that interface.
This could include the module procedures in a generic interface or the
implementation in a submodule of an interface in a parent module. Where possible, edges connecting nodes are given different colours to make them easier to distinguish in large graphs.
private function constructor(this_size, next_size) result(layer)
Layer class constructor. this_size is the number of neurons in the layer.
next_size is the number of neurons in the next layer, used to allocate
the weights.