refactor sbc funcs and tests

[janfb]

• Refactored the SBC module for better code organization and API clarity.
• Moved VIPosterior handling to the public API with a wrapper approach.
• Added helper functions for better maintainability.
• Enhanced tests with fixtures, dedicated test cases for parallelization and batch sampling, and improved assertions.
• All changes maintain backward compatibility.

[codecov]

Codecov Report

Attention: Patch coverage is 86.11111% with 5 lines in your changes missing coverage. Please review.

Project coverage is 78.89%. Comparing base (3e845bd) to head (b5f3e2d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
sbi/diagnostics/sbc.py	86.11%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1578      +/-   ##
==========================================
- Coverage   86.02%   78.89%   -7.13%     
==========================================
  Files         135      135              
  Lines       10754    10770      +16     
==========================================
- Hits         9251     8497     -754     
- Misses       1503     2273     +770     

Flag	Coverage Δ
unittests	78.89% <86.11%> (-7.13%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sbi/diagnostics/sbc.py	88.50% <86.11%> (-4.46%)	⬇️

... and 31 files with indirect coverage changes

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[janfb]

we always call it with reduce_fn(theta, x)

[janfb]

we used this argument only to check whether it's a VIPosterior. we now do this above in run_sbc so that this internal function can be called with just samples.