fix issue when coords for default dims are provided

[OriolAbril]

Description

Currently if coordinate values are provided for "default dims" they are ignored.
This is an issue for example in pymc where posterior and posterior predictive are
separate sampling steps and you can choose to thin the posterior if generating
posterior predictive samples is expensive as it prevents the original
draw/chain ids to be preserved as coordinate values of the posterior predictive.

Checklist

• Follows official PR format
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #2001 (01315ce) into main (9deaa52) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 01315ce differs from pull request most recent head f4bafb4. Consider uploading reports for the commit f4bafb4 to get more accurate results

@@            Coverage Diff             @@
##             main    #2001      +/-   ##
==========================================
- Coverage   90.85%   90.81%   -0.05%     
==========================================
  Files         114      114              
  Lines       12332    12332              
==========================================
- Hits        11204    11199       -5     
- Misses       1128     1133       +5     

Impacted Files	Coverage Δ
arviz/data/base.py	98.21% <100.00%> (ø)
arviz/__init__.py	87.50% <0.00%> (-12.50%)	⬇️

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