generative model for XP

Adrian Price-Whelan (Flatiron) and I worked out a form for a possible generative model for the ESA Gaia XP spectra and stellar parameters. The idea of going generative is that it should degrade at low signal-to-noise ratios more sensibly than discriminative models degrade. Discriminative models, as a reminder, find a function of the features that predict the labels. Generative models find a function of latents that predict both the features and the labels; labeling becomes an inverse or inference problem.