Optional McBackend support

[michaelosthege]

This PR implements optional support for McBackend based trace backends.

This builds on top of the refactorings from #6475 that decoupled the pm.sampling module from BaseTrace.

I split the addition of McBackend support into two commits, the first of which makes the necessary code changes without actually adding McBackend as a dependency for the test environments just to confirm that the optional importing works.

The mcbackend.RunMeta object that is created by the make_runmeta function contains several important metadata about the current model variables. This includes information such as whether a variable is deterministic or not. Related issues:

• Remember which variables are deterministic arviz-devs/arviz#420
• Add property to remember the list of free RVs arviz-devs/arviz#1748
• v4 <-> InferenceData integration #5160

Checklist

• Explain important implementation details 👆
• Make sure that the pre-commit linting/style checks pass.
• Link relevant issues (preferably in nice commit messages)
• Are the changes covered by tests and docstrings?
• Fill out the short summary sections 👇

New features

• pm.sample(trace=...) now accepts mcbackend.Backend instances.

[codecov]

Codecov Report

Merging #6510 (c659bc6) into main (14a10b7) will decrease coverage by 8.86%.
The diff coverage is 94.39%.

❗ Current head c659bc6 differs from pull request most recent head 345e3ce. Consider uploading reports for the commit 345e3ce to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6510      +/-   ##
==========================================
- Coverage   91.91%   83.06%   -8.86%     
==========================================
  Files          89      148      +59     
  Lines       14936    28018   +13082     
==========================================
+ Hits        13729    23272    +9543     
- Misses       1207     4746    +3539     

Impacted Files	Coverage Δ
pymc/tests/sampling/test_mcmc.py	0.00% <0.00%> (ø)
pymc/tests/step_methods/hmc/test_hmc.py	98.14% <ø> (ø)
pymc/tests/step_methods/test_compound.py	0.00% <0.00%> (ø)
pymc/backends/__init__.py	74.28% <78.57%> (-21.17%)	⬇️
pymc/backends/mcbackend.py	97.08% <97.08%> (ø)
pymc/tests/backends/test_mcbackend.py	97.72% <97.72%> (ø)
pymc/sampling/mcmc.py	72.88% <100.00%> (-19.25%)	⬇️
pymc/step_methods/compound.py	96.69% <100.00%> (-1.35%)	⬇️
pymc/printing.py	19.35% <0.00%> (-68.55%)	⬇️
pymc/step_methods/slicer.py	28.75% <0.00%> (-67.50%)	⬇️
... and 90 more

[michaelosthege]

Whoopsie, are you aware that the last commit changed the default for situations where McBackend is installed?
I mean it works for the CI, but it might be a little risky

[twiecki]

I don't think we should change the default yet.