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.

Resubmission of #6988

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--7185.org.readthedocs.build/en/7185/

[tvwenger]

@ricardoV94 other branch got mucked up during upstream rebase, so I closed that PR in favor of this one. It's ready for review when the checks are complete.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.26%. Comparing base (47f6d9e) to head (dfec0af).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7185      +/-   ##
==========================================
+ Coverage   92.24%   92.26%   +0.02%     
==========================================
  Files         100      100              
  Lines       16888    16880       -8     
==========================================
- Hits        15578    15574       -4     
+ Misses       1310     1306       -4     

Files	Coverage Δ
pymc/distributions/continuous.py	97.79% <100.00%> (+0.25%)	⬆️

... and 1 file with indirect coverage changes

[tvwenger]

I had to revert the behavior for negative tau since it broke other tests.

[tvwenger]

@ricardoV94 Ready to merge

[ricardoV94]

Thanks @tvwenger