Function build_training_inputs 

pub fn build_training_inputs(
    inputs: &TrainingInputs,
    generation: u64,
    embedding_model_id: &str,
    config: &TrainingConfig,
) -> Result<TrainingSnapshot, String>

Build the training snapshot from loaded inputs. Pure and slow (the per-person features are quadratic in a person’s face count), so it runs with no connection held.