Refactor get_tau_sigma and support lists of variables

[tvwenger]

What is this PR about?
This PR refactors the get_tau_sigma logic in order to support lists of Variables and fixes #6987.

N.B. The original code had a positivity check for non-Variable arguments, but not for Variable arguments. Since this PR casts the arguments to Variable, I've removed the positivity check for non-Variable arguments.

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

Major / Breaking Changes

• get_tau_sigma now always returns a Variable, even when scalars are passed

New features

• N/A

Bugfixes

• BUG: get_tau_sigma does not support list of variables #6987

Documentation

• N/A

Maintenance

• Updated tests

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📚 Documentation preview 📚: https://pymc--6988.org.readthedocs.build/en/6988/

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 77.73%. Comparing base (46f1675) to head (bad3b80).
Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6988       +/-   ##
===========================================
- Coverage   89.60%   77.73%   -11.88%     
===========================================
  Files         100      100               
  Lines       16888    16880        -8     
===========================================
- Hits        15133    13121     -2012     
- Misses       1755     3759     +2004     

Files	Coverage Δ
pymc/distributions/shape_utils.py	59.89% <ø> (-36.05%)	⬇️
pymc/model/core.py	72.15% <ø> (-19.18%)	⬇️
pymc/model_graph.py	14.57% <ø> (-62.82%)	⬇️
pymc/pytensorf.py	73.61% <ø> (-17.42%)	⬇️
pymc/distributions/continuous.py	64.64% <90.00%> (-32.90%)	⬇️

... and 40 files with indirect coverage changes

[ricardoV94]

@tvwenger would you like to update this PR? Would only need to add some regression tests for the original issue

[tvwenger]

@ricardoV94 Thanks for the reminder and sorry for the delay. I added some regression tests, so this should be good to go once the checks are finished.

[tvwenger]

@ricardoV94 This PR is ready for review

[ricardoV94]

This looks good, just two nitpicks, one of which is not related to your changes. Let me know if you want me to merge or if you want to address them.

[tvwenger]

@ricardoV94 Ready for review when checks are done