Comprehensive Configs: When saving the model, export the fullest Ludwig config that includes module-specific parameters

[justinxzhao]

The current "expanded" Ludwig config (saved as model_hyperparameters.json) contains user config-specified parameters merged with preprocesing and metadata parameters. However, there aren't any module-specific parameters (unless they were explicitly specified by the initial config).

When saving the model, we should export an even fuller Ludwig config that includes module-specific parameters. This allows older models to load safely if there's new default behavior within a module, like when we add a rule of thumb for embedding size.