Introduce Valued Variables in logprob IR

[ricardoV94]

This simplifies the rewrite logic quite a bit and fixes #6917

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:books: Documentation preview :books:: https://pymc--6918.org.readthedocs.build/en/6918/

[codecov[bot]]

Codecov Report

Merging #6918 (ff59803) into main (ec4407d) will decrease coverage by 24.59%. The diff coverage is 58.63%.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6918       +/-   ##
===========================================
- Coverage   87.78%   63.19%   -24.59%     
===========================================
  Files         100      100               
  Lines       16896    16736      -160     
===========================================
- Hits        14832    10577     -4255     
- Misses       2064     6159     +4095     

Files	Coverage Δ
pymc/distributions/mixture.py	33.61% <100.00%> (-61.37%)	:arrow_down:
pymc/distributions/multivariate.py	50.00% <100.00%> (-7.48%)	:arrow_down:
pymc/gp/cov.py	98.00% <100.00%> (ø)
pymc/gp/util.py	96.47% <100.00%> (+0.04%)	:arrow_up:
pymc/initial_point.py	100.00% <100.00%> (ø)
pymc/logprob/rewriting.py	93.47% <100.00%> (+3.36%)	:arrow_up:
pymc/model/core.py	64.97% <100.00%> (-26.78%)	:arrow_down:
pymc/model/fgraph.py	25.25% <100.00%> (-71.65%)	:arrow_down:
pymc/model/transform/conditioning.py	17.20% <100.00%> (-78.49%)	:arrow_down:
pymc/logprob/censoring.py	63.52% <75.00%> (-35.35%)	:arrow_down:
... and 14 more

... and 38 files with indirect coverage changes