Refactoring towards IBaseTrace interfaces

[michaelosthege]

The goal of this PR is to identify & specify the BaseTrace attributes & methods that are necessary from the perspectives of internal sampling functions (pm.sampling.mcmc.*) and MultiTrace.

By extracting these signatures/interfaces into an abstract class IBaseTrace it becomes much easier to write a small adapter class that can wrap other kinds of backends to be used instead of BaseTrace. (hint hint 🍔)

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 👇

Major / Breaking Changes

• MultiTrace.straces type changes from Sequence[BaseTrace] to Sequence[IBaseTrace]

Maintenance

• Internal backend types and sampling functions are refactored for more substitutability
• Fixes flakyness of the weibull_logp test

[codecov]

Codecov Report

Merging #6475 (b0a960b) into main (00d2675) will decrease coverage by 0.01%.
The diff coverage is 82.92%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6475      +/-   ##
==========================================
- Coverage   94.79%   94.79%   -0.01%     
==========================================
  Files         148      148              
  Lines       27730    27745      +15     
==========================================
+ Hits        26287    26301      +14     
- Misses       1443     1444       +1     

Impacted Files	Coverage Δ
pymc/backends/base.py	84.97% <73.07%> (+0.15%)	⬆️
pymc/backends/__init__.py	95.45% <100.00%> (+1.33%)	⬆️
pymc/backends/ndarray.py	79.27% <100.00%> (ø)
pymc/sampling/mcmc.py	93.06% <100.00%> (ø)
pymc/tests/distributions/test_continuous.py	99.77% <100.00%> (+<0.01%)	⬆️

[aloctavodia]

LGTM