Implement new model

After adding the non linear decoder, its time to go to the next level.

The issue references a 'non linear decoder' and moving to 'the next level', but provides no specifics on what the new model should be, what files to modify, or what 'done' looks like. A newcomer would need to understand the existing VAE implementation in the repository first, then propose a design for the new model.

索引モデルが issue の本文から書いたものです。