fix: posterior potential iid handling

[janfb]

Somehow the iid handling for NPE was failing because is was not checking for the shape > 1, but directly failing when x_is_iid=True.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.37%. Comparing base (299854e) to head (f9dfd85).
Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1276       +/-   ##
===========================================
- Coverage   89.24%   78.37%   -10.88%     
===========================================
  Files         119      119               
  Lines        8695     8698        +3     
===========================================
- Hits         7760     6817      -943     
- Misses        935     1881      +946     

Flag	Coverage Δ
unittests	78.37% <100.00%> (-10.88%)	⬇️

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Files with missing lines	Coverage Δ
.../inference/potentials/posterior_based_potential.py	95.34% <100.00%> (ø)

... and 31 files with indirect coverage changes