Refactor RatioEstimator to subclass ConditionalEstimator and remove redundant builder protocols

[abelaba]

This PR updates the RatioEstimator class to be a subclass of ConditionalEstimator. It also removes the RatioEstimatorBuilder and VectorFieldEstimatorBuilder protocols, as these protocols can be represented using the DensityEstimatorBuilder protocol.

[codecov]

Codecov Report

❌ Patch coverage is 94.11765% with 2 lines in your changes missing coverage. Please review.
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Files with missing lines	Patch %	Lines
sbi/diagnostics/misspecification.py	50.00%	1 Missing ⚠️
sbi/neural_nets/ratio_estimators.py	88.88%	1 Missing ⚠️

Additional details and impacted files

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Flag	Coverage Δ
unittests	82.56% <94.11%> (?)

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Files with missing lines	Coverage Δ
sbi/inference/trainers/base.py	93.52% <100.00%> (ø)
sbi/inference/trainers/nle/mnle.py	95.65% <100.00%> (ø)
sbi/inference/trainers/nle/nle_a.py	100.00% <100.00%> (ø)
sbi/inference/trainers/nle/nle_base.py	97.82% <100.00%> (ø)
sbi/inference/trainers/npe/mnpe.py	95.65% <100.00%> (ø)
sbi/inference/trainers/npe/npe_a.py	63.91% <ø> (ø)
sbi/inference/trainers/npe/npe_b.py	100.00% <100.00%> (ø)
sbi/inference/trainers/npe/npe_base.py	94.77% <100.00%> (ø)
sbi/inference/trainers/npe/npe_c.py	73.33% <ø> (ø)
sbi/inference/trainers/nre/bnre.py	46.66% <100.00%> (ø)
... and 10 more

[janfb]

this is a great refactoring @abelaba! 👏
if we reconcile all the protocols into one then this should not be called DensityEstimatorBuilder because it also concerns functions that build ratio estimators or vector field estimators. I suggest to do a renaming into ConditionalEstimatorBuilder.

this is the only required change for now, I will do another full review afterwards - thanks! 🙏

[janfb]

too small things to fix, otherwise good to go 🚀

[janfb]