Macro impl_config_new_and_with_common 

macro_rules! impl_config_new_and_with_common {
    (
        $Config:ident,
        common_defaults: ($model_name_opt:expr, $batch_size_opt:expr),
        fields: { $( $field:ident : $default_expr:expr ),* $(,)? }
    ) => { ... };
}

Macro to implement new() and with_common() for config structs with per-module defaults.