Build NIMBLE model code from a compiled model spec
build_code_from_spec.RdDispatches to the backend-specific code generators:
build_code_sb_from_spec()for stick-breaking ("sb")build_code_crp_from_spec()for CRP and spliced ("crp","spliced")
build_code_from_spec.RdDispatches to the backend-specific code generators:
build_code_sb_from_spec() for stick-breaking ("sb")
build_code_crp_from_spec() for CRP and spliced ("crp", "spliced")