Add a better error message for how to debug "Cannot find valid initial parameters" issue

[fehiepsi]

Recently, several users got this issue but it is not easy to diagnose the issue just by looking at the model. We should add a better error message, e.g. to mention about numpyro.validation_enabled() utility, which tells us at which latent site, we get invalid parameters/values.

[fehiepsi]

We can also run the model with validation_enabled once if we can't find a valid initial parameters.