Base estimator class

[manuelgloeckler]

What does this implement/fix? Explain your changes

Implements an abstract base class for estimators, which has:

• input_shape
• condition_shape
• abstract loss function.

Does this close any currently open issues?

Will close #966.
Will replace #1072 .

Considerations:

• Density estimator sample and log_prob could be abstract methods, instead of raising a NotImplementedError

[codecov]

Codecov Report

Attention: Patch coverage is 62.06897% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 76.98%. Comparing base (afbd5e7) to head (17c5343).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1151      +/-   ##
==========================================
- Coverage   85.08%   76.98%   -8.10%     
==========================================
  Files          90       90              
  Lines        6643     6662      +19     
==========================================
- Hits         5652     5129     -523     
- Misses        991     1533     +542     

Flag	Coverage Δ
unittests	76.98% <62.06%> (-8.10%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
sbi/neural_nets/density_estimators/base.py	61.70% <62.06%> (+4.55%)	⬆️

... and 23 files with indirect coverage changes