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 https://github.com/pymc-devs/pymc/issues/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 https://github.com/pymc-devs/pymc/pull/6988

Checklist

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

• https://github.com/pymc-devs/pymc/issues/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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

[](https://app.codecov.io/gh/pymc-devs/pymc/pull/7185?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymc-devs) ```diff @@ 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](https://app.codecov.io/gh/pymc-devs/pymc/pull/7185?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymc-devs) | Coverage Δ | | |---|---|---| | [pymc/distributions/continuous.py](https://app.codecov.io/gh/pymc-devs/pymc/pull/7185?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymc-devs#diff-cHltYy9kaXN0cmlidXRpb25zL2NvbnRpbnVvdXMucHk=) | `97.79% <100.00%> (+0.25%)` | :arrow_up: | ... and [1 file with indirect coverage changes](https://app.codecov.io/gh/pymc-devs/pymc/pull/7185/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pymc-devs)

[tvwenger]

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

[tvwenger]

@ricardoV94 Ready to merge

[ricardoV94]

Thanks @tvwenger