Misc fixes for default-constructibility of our learners

[timokau]

Description

Pulling out the changes of https://github.com/kiudee/cs-ranking/pull/116 that are ready for merge now. As I said in https://github.com/kiudee/cs-ranking/pull/116#issuecomment-650589268, this brings us very close to passing the "default-constructible" test. I think/hope that will be the highest hurdle for sklearn api compatibility.

Outstanding issues are the kernel-regularizers and https://github.com/scikit-learn/scikit-learn/issues/17756.

Motivation and Context

See #116.

How Has This Been Tested?

Linters & tests.

Does this close/impact existing issues?

Impacts #116, but doesn't close it.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.

[timokau]

Removing the regularizer default overrides is a small breaking change (analogous to #142).

[codecov[bot]]

Codecov Report

Merging #144 into master will increase coverage by 0.03%. The diff coverage is 70.73%.

@@            Coverage Diff             @@
##           master     #144      +/-   ##
==========================================
+ Coverage   60.08%   60.11%   +0.03%     
==========================================
  Files         116      116              
  Lines        7662     7668       +6     
==========================================
+ Hits         4604     4610       +6     
  Misses       3058     3058              

Impacted Files	Coverage Δ
csrank/choicefunction/cmpnet_choice.py	84.78% <ø> (ø)
csrank/choicefunction/fate_choice.py	91.30% <ø> (ø)
csrank/choicefunction/ranknet_choice.py	81.63% <ø> (ø)
csrank/core/cmpnet_core.py	89.28% <ø> (ø)
csrank/core/fate_network.py	69.19% <ø> (ø)
csrank/core/feta_linear.py	92.24% <ø> (ø)
csrank/core/ranknet_core.py	88.69% <ø> (ø)
csrank/discretechoice/cmpnet_discrete_choice.py	86.84% <ø> (ø)
csrank/discretechoice/fate_discrete_choice.py	94.44% <ø> (ø)
csrank/discretechoice/ranknet_discrete_choice.py	84.61% <ø> (ø)
... and 14 more

[timokau]

Code coverage is only decreasing because of the black changes and doc additions.

[timokau]

Thanks for the feedback, I addressed all your comments. Please have another look.