get_inits not working with vb output

Summary:

Please provide a short summary (no more than a sentence or two).

get_inits is not working correctly for the output of vb on at least one of the example models.

Description:

Describe the issue as clearly as possible.

The output of get_inits applied to vb output does not match the declared parameter dimensions of the model. A simple script that reproduces the problem is below. Note that the output of get_inits has the wrong dimension for sigma, and cannot be passed to unconstrain_pars, whereas the same commands work for ordinary sampling.

Reproducible Steps:

Please report steps to reproduce the issue. If it's not possible to reproduce, please include a description of how you discovered the issue.

If you have a reproducible example, please include it.

library(rstan)
options(mc.cores=4)

rstan_options(auto_write=TRUE)

# In example-models/ARM/Ch.17 on your local computer
setwd("/home/rgiordan/Documents/git_repos/example-models/ARM/Ch.17")
model <- stan_model("17.4_multilevel_logistic.stan")

# Load the data and hyperparameters.
stan_data <- new.env()
source("17.4_multilevel_logistic.data.R", local=stan_data)
stan_data <- as.list(stan_data)

num_samples <- 1000
chains <- 1
sampling_result <- sampling(model, data=stan_data, chains=chains, iter=num_samples)
advi_result <- vb(model, data=stan_data, iter=num_samples)

draw_list <- get_inits(sampling_result, iter=1)[[1]]
length(draw_list$sigma) # should be 1, and is 1
unconstrain_pars(sampling_result, draw_list) # Works as expected

vb_draw_list <- get_inits(advi_result, iter=1)
length(vb_draw_list$sigma)  # should be 1, and is 93
unconstrain_pars(advi_result, vb_draw_list) # fails with the message:
# Error in object@.MISC$stan_fit_instance$unconstrain_pars(pars) : 
#  mismatch in number dimensions declared and found in context; processing stage=initialization; variable 
# name=sigma; dims declared=(); dims found=(93)

Current Output:

If applicable, any relevant output from RStan.

Should be clear from the above example.

Expected Output:

If applicable, the output you expected from RStan.

Should be clear from the above example.

RStan Version:

The version of RStan you are running (e.g., from packageVersion("rstan"))

2.17.3

R Version:

The version of R you are running (e.g., from R.version.string)

R version 3.4.4 (2018-03-15)

Operating System:

Your operating system (e.g., OS X 10.11.3)

Ubuntu 16.04

Start by running the provided R reproduction with example-models/ARM/Ch.17/17.4_multilevel_logistic.stan, comparing get_inits output from sampling and vb. Trace how get_inits handles vb_draw_list and check the sigma dimensions passed to unconstrain_pars; done means vb output matches the declared dimensions and unconstrain_pars succeeds.