Materialize anonymous variables later only if needed

[bvdmitri]

Includes #163 (please merge #163 first)

A silly bug we missed, the fix is pretty easy though.

Didn't add the tests yet (added), but this is a reproducer

@model function mv_iid_inverse_wishart_known_mean(y, m, d)
    C ~ InverseWishart(d + 1, diageye(d))

    for i in eachindex(y)
        y[i] ~ MvNormal(μ = m, Σ = C)
    end
end

Without this PR the engine creates an anonymous random variable for d + 1, but it shouldn't (if d is a number and not a data).

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 91.34%. Comparing base (9de8055) to head (e199e5b).
Report is 25 commits behind head on dev-4.0.0.

Files	Patch %	Lines
src/graph_engine.jl	95.23%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff              @@
##           dev-4.0.0     #164      +/-   ##
=============================================
+ Coverage      90.98%   91.34%   +0.35%     
=============================================
  Files             14       14              
  Lines           1742     1733       -9     
=============================================
- Hits            1585     1583       -2     
+ Misses           157      150       -7     

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