Fix Flat/HalfFlat initial value dimensionality

[michaelosthege]

Just adding the MRE as unit tests.

Anybody please feel free to pick this up.

[codecov]

Codecov Report

Merging #4996 (30040cd) into main (59ce9af) will decrease coverage by 3.06%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4996      +/-   ##
==========================================
- Coverage   76.34%   73.28%   -3.07%     
==========================================
  Files          86       86              
  Lines       13931    13931              
==========================================
- Hits        10636    10209     -427     
- Misses       3295     3722     +427     

Impacted Files	Coverage Δ
pymc3/printing.py	22.22% <0.00%> (-63.64%)	⬇️
pymc3/distributions/bound.py	40.00% <0.00%> (-60.00%)	⬇️
pymc3/distributions/discrete.py	79.00% <0.00%> (-20.00%)	⬇️
pymc3/distributions/multivariate.py	54.31% <0.00%> (-17.71%)	⬇️
pymc3/distributions/continuous.py	84.11% <0.00%> (-12.16%)	⬇️
pymc3/distributions/dist_math.py	81.28% <0.00%> (-6.96%)	⬇️
pymc3/distributions/distribution.py	78.00% <0.00%> (-4.67%)	⬇️
pymc3/distributions/logprob.py	94.52% <0.00%> (-1.37%)	⬇️
pymc3/distributions/transforms.py	91.30% <0.00%> (-1.09%)	⬇️
pymc3/math.py	67.17% <0.00%> (-1.02%)	⬇️
... and 1 more

[michaelosthege]

Superseded by #4983 which is going to avoid the issue alltogether.