`metabayesian_MH` is not a meanful model

[yebai]

Differentiating through an MCMC sampler as below is an uncommon operation.

ADTests/models/metabayesian_MH.jl

Lines 32 to 36 in ff99b21

	inner_m = inner_model(observation, subj_prior_μ, subj_prior_σ)
	# Run the inner Bayesian inference
	chns = sample(Xoshiro(468), inner_m, inner_sampler, inner_n_samples, progress = false)
	# Extract subjective point estimate
	subj_mean_expectationₜ = mean(chns[:mean])

I don't think it is a meaningful example for AD tests since most AD backends won't have a rule for Turing.sample. Meanwhile, automatically derived rules for Turing.sample are meaningless for most MCMC samplers, including MH.