feat: assign weights in graph

[miparnisari]

Description

This is the scaffolding code required to compute weights for each node and edge. It's not meant to support every kind of model - more followup PRs will come. (For example, models with cycles are not handled).

Please review each model in the weighted_graph_builder_test.go and visualize the output in https://dreampuf.github.io/GraphvizOnline/. This will help you review the correctness of the weight assignment algorithm.

Example