Causal T-learner

[eduardodx]

Status

IN DEVELOPMENT

• IN DEVELOPMENT: there's active development being done in this branch, don't merge yet
• HOLD: development is done, but PR shouldn't be merged yet (e.g. a correspondent PR in another service needs to be merged first)
• READY: development is over and everything is ready for merging (leave only one of the options above)

Todo list

• [x] Documentation
• [x] Tests added and passed
• [ ] Issue: closes #xxxx (write the number of the GitHub issues that this PR resolves)

Background context

This PR adds another causal model, the T-Learner, to fklearn, based on another merged branch: causal-s-learner

Description of the changes proposed in the pull request

Updates the meta_learners.py

Where should the reviewer start?

A good start is reviewing the causal_t_classification_learner function code and, as you're ready, go through the auxiliary functions.

[codecov-commenter]

Codecov Report

Merging #206 (17d4d26) into master (3cd7bec) will decrease coverage by 0.50%. The diff coverage is 92.92%.

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
- Coverage   94.69%   94.18%   -0.51%     
==========================================
  Files          25       34       +9     
  Lines        1507     2097     +590     
  Branches      203      274      +71     
==========================================
+ Hits         1427     1975     +548     
- Misses         48       83      +35     
- Partials       32       39       +7     

Impacted Files	Coverage Δ
src/fklearn/causal/validation/cate.py	0.00% <0.00%> (ø)
src/fklearn/data/datasets.py	100.00% <ø> (ø)
src/fklearn/tuning/parameter_tuners.py	79.48% <ø> (ø)
src/fklearn/tuning/selectors.py	90.47% <ø> (ø)
src/fklearn/validation/validator.py	88.88% <71.42%> (-5.40%)	:arrow_down:
src/fklearn/preprocessing/splitting.py	95.00% <92.59%> (-0.84%)	:arrow_down:
src/fklearn/causal/cate_learning/meta_learners.py	92.85% <92.85%> (ø)
src/fklearn/training/calibration.py	96.36% <94.73%> (-3.64%)	:arrow_down:
src/fklearn/training/transformation.py	93.95% <95.12%> (+0.02%)	:arrow_up:
src/fklearn/validation/evaluators.py	93.95% <96.29%> (+4.32%)	:arrow_up:
... and 18 more

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[prcastro]

Just need to add some tests