Refactoring and addition of helpers to handle flat stats

[michaelosthege]

This is one step towards refactoring of the trace backend.

PyMC supports hierarchies of samplers, which means that sampler stats are not a flat list by default.

First I moved Competence and BlockedStep to the compound module to avoid a circular import.

Second, I added a StatsBijection which can be used to flatten a list of sampler stats dicts into one dictionary.

Related: #4602

Checklist

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Maintenance

• Competence and BlockedStep were moved to the compound module.
• A non-public StatsBijection was added.

[codecov]

Codecov Report

Merging #6443 (20a858d) into main (434333f) will increase coverage by 8.84%.
The diff coverage is 97.72%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6443      +/-   ##
==========================================
+ Coverage   85.92%   94.77%   +8.84%     
==========================================
  Files         148      148              
  Lines       27691    27753      +62     
==========================================
+ Hits        23794    26302    +2508     
+ Misses       3897     1451    -2446     

Impacted Files	Coverage Δ
pymc/step_methods/compound.py	97.43% <96.70%> (-2.57%)	⬇️
pymc/blocking.py	95.65% <100.00%> (+0.09%)	⬆️
pymc/step_methods/arraystep.py	94.52% <100.00%> (-0.22%)	⬇️
pymc/step_methods/hmc/hmc.py	92.85% <100.00%> (ø)
pymc/step_methods/hmc/nuts.py	97.29% <100.00%> (ø)
pymc/step_methods/metropolis.py	83.79% <100.00%> (+0.03%)	⬆️
pymc/step_methods/slicer.py	96.25% <100.00%> (+0.04%)	⬆️
pymc/tests/step_methods/test_compound.py	100.00% <100.00%> (ø)
pymc/sampling/parallel.py	88.42% <0.00%> (+1.05%)	⬆️
pymc/logprob/cumsum.py	100.00% <0.00%> (+3.12%)	⬆️
... and 23 more