Add variances dispatch for distributions

[jessegrabowski]

Description

This PR adds a variance function that returns the (symbolic) variance of a distribution. I added it because I thought it would be nice to have for the AutoGuide stuff; we can use the mean and variance of priors to initialize the parameters.

This is a 90%+ LLM PR. I went through and tried to clean things up. I mostly copied the patterns from mean.py.

Related Issue

• Closes #
• Related to Add mean dispatch for pymc distributions #7530

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):

[codecov]

Codecov Report

❌ Patch coverage is 98.23009% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.97%. Comparing base (5ff12c8) to head (9649395).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/distributions/moments/variances.py	98.21%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8084      +/-   ##
==========================================
+ Coverage   90.80%   90.97%   +0.16%     
==========================================
  Files         121      124       +3     
  Lines       19443    19714     +271     
==========================================
+ Hits        17656    17935     +279     
+ Misses       1787     1779       -8     

Files with missing lines	Coverage Δ
pymc/distributions/moments/__init__.py	100.00% <100.00%> (ø)
pymc/distributions/moments/variances.py	98.21% <98.21%> (ø)

... and 12 files with indirect coverage changes

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