fix: score imports

[janfb]

add score classes to init to avoid circular imports.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.16%. Comparing base (b46c7c5) to head (679a8fe).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1246      +/-   ##
==========================================
- Coverage   86.05%   78.16%   -7.90%     
==========================================
  Files         117      118       +1     
  Lines        8672     8679       +7     
==========================================
- Hits         7463     6784     -679     
- Misses       1209     1895     +686     

Flag	Coverage Δ
unittests	78.16% <100.00%> (-7.90%)	⬇️

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Files with missing lines	Coverage Δ
sbi/inference/posteriors/score_posterior.py	95.38% <100.00%> (-1.64%)	⬇️
sbi/samplers/score/__init__.py	100.00% <100.00%> (ø)
sbi/samplers/score/score.py	84.90% <100.00%> (-0.28%)	⬇️

... and 27 files with indirect coverage changes