Fix compute_log_prior in models with Deterministics

[OriolAbril]

Description

The default for var_names in compute_log_likelihood was unobserved_RVs which
includes also deterministics. This changes the default to free_RVs.

Related Issue

• Closes #
• 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
• 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--7168.org.readthedocs.build/en/7168/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (97a7a00) 92.29% compared to head (34d2a5d) 92.29%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7168   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files         101      101           
  Lines       16947    16947           
=======================================
  Hits        15642    15642           
  Misses       1305     1305           

Files	Coverage Δ
pymc/stats/log_density.py	98.30% <100.00%> (ø)