Implementing Variational Autoencoder using older MATLAB versions

Hi! I have come across the following example to implement Variational Autoencoders (VAE):

https://www.mathworks.com/help/deeplearning/examples/train-a-variational-autoencoder-vae-to-generate-images.html

However, it uses latest functions available in MATLAB R2019b like 'dlnetwork' and features like Custom Training Loop and automatic differentiation, etc. Any guidance would be appreciated regarding implementing VAE using older versions of MATLAB (before 2019b) and the Deep Learning Toolboox? Thanks & regards