Relax pm.observe to allow observing already observed variables

[zaxtax]

Description

This PR closes #6932 by relaxing checks over which model variables can be observed to include already observed variables.

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--7679.org.readthedocs.build/en/7679/

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.

Project coverage is 92.70%. Comparing base (d7d2be2) to head (02bb237).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pymc/model/transform/conditioning.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7679   +/-   ##
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  Coverage   92.70%   92.70%           
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  Files         107      107           
  Lines       18391    18391           
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  Hits        17050    17050           
  Misses       1341     1341           

Files with missing lines	Coverage Δ
pymc/model/transform/conditioning.py	95.74% <66.66%> (ø)

[ricardoV94]

The message below should be updated to "... is not a random variable or deterministic in the model"

You can use model.basic_RVs in place of free + observed, and ModelValuedVar for the corresponding in the ir representation in the assert below