Fix independent_rvs determination in vectorize_over_posterior

[lucianopaz]

Description

This PR fixes the determination of independent_rvs RVs in vectorize_over_posterior. I think that the fixed check is much clearer now. It says that an rv is independent if it doesn't depend on an output, a needed_rv (a model freeRV that is replaced by its posterior samples), or another independent_rv. To make sure the check works correctly, the candidate rv nodes are searched in the order given by their topological sort.

Related Issue

• Closes vectorize_over_posterior fails to determine independent_rvs #7889
• Related to #

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):

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📚 Documentation preview 📚: https://pymc--7890.org.readthedocs.build/en/7890/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (0960323) to head (dcb68e6).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7890   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files         116      116           
  Lines       18851    18851           
=======================================
+ Hits        17521    17522    +1     
+ Misses       1330     1329    -1     

Files with missing lines	Coverage Δ
pymc/sampling/forward.py	96.73% <100.00%> (ø)

... and 1 file with indirect coverage changes

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