Add icdf functions for Inverse Gamma distribution

[lucaseckes]

Adds ICDF functions to Inverse Gamma distribution

Description

The quantile function depends on the quantile function of Gamma distribution: #6845

Related Issue

• Closes #
• Related to Add icdf functions for distributions #6612

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):

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📚 Documentation preview 📚: https://pymc--7917.org.readthedocs.build/en/7917/

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.63%. Comparing base (340e403) to head (45b0b36).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7917   +/-   ##
=======================================
  Coverage   91.62%   91.63%           
=======================================
  Files         116      116           
  Lines       18852    18856    +4     
=======================================
+ Hits        17274    17278    +4     
  Misses       1578     1578           

Files with missing lines	Coverage Δ
pymc/distributions/continuous.py	98.26% <100.00%> (+<0.01%)	⬆️

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[jessegrabowski]

Looks good to me! @ricardoV94 do you want to check it before I snap merge?

[ricardoV94]

Better to use the stable API, and then you don't need to hack with the scale thing either

Suggested change

	res = 1 / Gamma.icdf(value=1 - value, alpha=alpha, scale=1 / beta)
	res = 1 / icdf(Gamma.dist(alpha=alhpa, beta=beta), value=1 - value)

[ricardoV94]

For curiosity only, any intuition why the icdf looks like it does, specially the flipped value. It's interesting