Missing samplers for a variable doesn't error

[penelopeysm]

Minimal working example

@model function gdemo(x, y)
    s² ~ InverseGamma(2, 3)
    m ~ Normal(0, sqrt(s²))
    x ~ Normal(m, sqrt(s²))
    y ~ Normal(m, sqrt(s²))
end
chain = sample(
    gdemo(1.5, 2.0),
    MH(:m => Normal(0, 1)),
    10
)

Description

s² is sampled once at the start of the chain and isn't touched again. More intuitive behaviour would be to error.

cf. discussion in #2360

julia> chain[:s²]
2-dimensional AxisArray{Float64,2,...} with axes:
    :iter, 1:1:10
    :chain, 1:1
And data, a 10×1 Matrix{Float64}:
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377
 2.1037496343703377

Julia version info

versioninfo()

julia> versioninfo()
Julia Version 1.10.5
Commit 6f3fdf7b362 (2024-08-27 14:19 UTC)
Build Info:
  Official https://julialang.org/ release
Platform Info:
  OS: macOS (arm64-apple-darwin22.4.0)
  CPU: 10 × Apple M1 Pro
  WORD_SIZE: 64
  LIBM: libopenlibm
  LLVM: libLLVM-15.0.7 (ORCJIT, apple-m1)
Threads: 1 default, 0 interactive, 1 GC (on 8 virtual cores)

Manifest

]st --manifest

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