Fix bug with `compute_p` for partially observed OrderedLogistic and OrderedProbit variables

[ricardoV94]

This fixes a bug when auto-imputation takes place, in which case the logic to retrieve p wasn't valid. This will also show p as an input of the underlying Categorical in the graphviz.

Description

This PR fixes a bug when setting compute_p=True for partially observed OrderedLogistic/Probit. The logic of retrieving p from the third input does not work in this case, because when there is imputation a different kind of variable is returned (the joined vector of imputed and observed variables)

Checklist

• [x] Checked that the pre-commit linting/style checks pass
• [x] Included tests that prove the fix is effective or that the new feature works
• [x] Added necessary documentation (docstrings and/or example notebooks)
• [x] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [x] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7349.org.readthedocs.build/en/7349/