Assemble NUTS initial positions and an inverse-mass diagonal

Usage

.build_warm_start(
  fit,
  layout,
  re_terms,
  n_chains,
  sigma_re = NULL,
  jitter = 1,
  metric = FALSE
)

fit: Source fit (mode = "eb" or "laplace").
layout: The target sampler layout, from cpp_tulpa_glmm_layout().
re_terms: Random-effect terms of the model being sampled.
n_chains: Number of chains to initialise.
sigma_re: Random-effect SDs, when the source fit conditioned on them.
jitter: Scale of the between-chain dispersion, as a multiple of each parameter's warm-start SD. 0 stacks every chain on the mode.
metric: Hand the assembled inverse-mass diagonal to the kernel as well as the position. FALSE (default) because it can only be filled for some blocks today and a partial metric measurably slows the sampler; see the note at the end of this function.

list(init = <n_chains x D matrix>, inv_metric_diag = <D vector or NULL>).