Allow creating CustomDist inside another CustomDist

[ricardoV94]

Fixes issue describe in this discourse thread: https://discourse.pymc.io/t/dynamic-shaping-round-function-jax-and-a-few-more-questions/12264/17?u=ricardov94

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📚 Documentation preview 📚: https://pymc--6822.org.readthedocs.build/en/6822/

[codecov]

Codecov Report

Merging #6822 (4866992) into main (f67ff8b) will decrease coverage by 4.37%.
The diff coverage is 100.00%.

❗ Current head 4866992 differs from pull request most recent head 3b97c29. Consider uploading reports for the commit 3b97c29 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6822      +/-   ##
==========================================
- Coverage   92.02%   87.66%   -4.37%     
==========================================
  Files          95       95              
  Lines       16261    16266       +5     
==========================================
- Hits        14964    14259     -705     
- Misses       1297     2007     +710     

Impacted Files	Coverage Δ
pymc/distributions/distribution.py	97.06% <100.00%> (ø)
pymc/model.py	91.00% <100.00%> (+0.05%)	⬆️

... and 7 files with indirect coverage changes

[larryshamalama]

Looks good! A more general question from my side: when do we want to use BlockModelAccess, i.e. prevent user access to Model contexts? Can we specify this in the BlockModelAccess docstring?

[ricardoV94]

Looks good! A more general question from my side: when do we want to use BlockModelAccess, i.e. prevent user access to Model contexts? Can we specify this in the BlockModelAccess docstring?

I think that's context specific. There may be several reasons to block. The only application so far is in CustomDist to make sure users don't accidentally write pm.Normal("x") instead of pm.Normal.dist() which would otherwise result in a very silent bug: introducing useless variables in the model every time the distribution is (re)created.