Fix #506 (missing logits_projector serialization)

[elseml]

Minimal fix providing missing serialization of the final logits_projector layer handled by the ModelComparisonApproximator class itself. Both the weights and the predictions of an original and a reloaded approximator of the example in #506 are now exactly equal. Please let me know if any further adaptations for other cases are needed.

As a more general discussion point: As there were some issues related to saving/loading recently, should we add some tests (e.g., using tempfiles) for the approximators that ensure that the weights and predictions of an original and a reloaded approximator match?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
...low/approximators/model_comparison_approximator.py	81.81% <100.00%> (ø)

[stefanradev93]

Thx, Lasse! This was mainly due to the way the build() function was handled before the stanradize_an_approx branch. I will make sure in a bit that the error is also resolved there.