allow compile time constraints in switch/join/make_vector logp rewrites

[eclipse1605]

Description

i relaxed the logp derivation restrictions for mixing measurable variables with compile time constants in branching/stacking ops. expressions like pt.where(cond, rv, -1) now produce a logp without requiring users to manually wrap constants in pm.DiracDelta.

notes

• only compile time constants are accepted
• only constant branches are allowed to broadcast.

Related Issue

• Closes #7711

Checklist

• Checked that the pre-commit linting/style checks pass
• Included tests that prove the fix is effective or that the new feature works
• Added necessary documentation (docstrings and/or example notebooks)
• 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):

[welcome]

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