Align autogenerated dimension names when dims and default_dims are provided

[lucianopaz]

Description

This PR closes pymc-devs/pymc#7572 and also #2397

There was a problem with auto generated dimension names for sample_posterior_predictive (but not for sample_prior_predictive). The only difference was that one code branch provided sample_dims and the other didn't, which resulted in having a list of dims in generate_dims_coords. This led to incorrect imputation of the dimension name.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2395.org.readthedocs.build/en/2395/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.81%. Comparing base (0868c9e) to head (cc6c545).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2395   +/-   ##
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  Coverage   86.81%   86.81%           
=======================================
  Files         124      124           
  Lines       12883    12883           
=======================================
  Hits        11184    11184           
  Misses       1699     1699           

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