the way the model is currently set up the
pyro.sample statement on line 212 takes a fixed
obs keyword which clamps the sample to the observed data. so if you remove that
obs keyword, the sample statement will in fact draw a sample. if you collect those samples you have a sampled sequence of x's. so that's how you would draw unconditionally from the model.
drawing from the model conditionally is a bit more complicated. that's because doing so would require using both the model and the guide. basically you use the guide to consume the sequence you'd like to condition on. then, at that point, you pass control over to the model, making sure that the first
z_prev seen by the model is the final
z sampled by the guide. if you have trouble getting this to work, i suggest you paste a code snippet that demonstrates your approach, and we can offer more concrete advice.