Fix spurious inputs failure when building Truncated variables

[ricardoV94]

Description

Reintroduce dummy intermediate variables in implementation of TruncatedRV

Partially reverts 9d4a3d7d371eda91be99c9bffc519d680d2cf290 and 3888d53575dfc2bf8527e4d69a7621645df5cbf6

The logprob derivation(s) in the icdf implementation of Truncated can duplicate nodes and cause spurious input variables to be marked as missing. We revert back to the old approach of replacing these by dummies so the graph above any inputs is hidden, and variables cannot be accidentally cloned/modified during logprob inference.

Related Issue

• [x] Closes #7312

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
• [ ] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [ ] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7328.org.readthedocs.build/en/7328/