Introduce sample_with="direct" for SNPE

[michaeldeistler]

API change: For SNPE, one now has to use direct instead of rejection if one wants to directly sample from the density estimator. direct is also the new default.

Closes #918

[codecov]

Codecov Report

Attention: Patch coverage is 33.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 76.00%. Comparing base (b856c07) to head (63386c1).

Files	Patch %	Lines
sbi/inference/snpe/snpe_base.py	33.33%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #942      +/-   ##
==========================================
- Coverage   76.04%   76.00%   -0.04%     
==========================================
  Files          80       80              
  Lines        6329     6331       +2     
==========================================
- Hits         4813     4812       -1     
- Misses       1516     1519       +3     

Flag	Coverage Δ
unittests	76.00% <33.33%> (-0.04%)	⬇️

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[janfb]

looks good, thanks for fixing this 🥇

todo: tests are failing because of black