Refactor BaseLabeler as parent class of label models

[vincentschen]

Description of proposed changes

• Move predict_proba to BaseLabeler class as abtractmethod
• Move predict, score, save, and load methods to BaseLabeler class
  as shared methods in parent class
• Update RandomVoter, MajorityClassVoter, MajorityLabelVoter, and
  LabelModel to be subclasses of BaseLabeler.
• Update docstrings to reflect abstract/parent class hierarchy.
• NOTE: We re-define methods in the LabelModel(BaseLabeler) subclass for predict and score in order to explicitly show docstring examples

Test plan

• Existing tests pass: Ran tox and tox -e complex.

Checklist

Need help on these? Just ask!

• I have read the CONTRIBUTING document.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have run tox -e complex and/or tox -e spark if appropriate.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #1559 into master will increase coverage by 0.01%.
The diff coverage is 97.56%.

@@            Coverage Diff             @@
##           master    #1559      +/-   ##
==========================================
+ Coverage   97.11%   97.13%   +0.01%     
==========================================
  Files          55       56       +1     
  Lines        2079     2091      +12     
  Branches      342      342              
==========================================
+ Hits         2019     2031      +12     
  Misses         31       31              
  Partials       29       29              

Impacted Files	Coverage Δ
snorkel/labeling/model/base_labeler.py	96.96% <96.96%> (ø)
snorkel/labeling/model/baselines.py	100.00% <100.00%> (+2.85%)	⬆️
snorkel/labeling/model/label_model.py	95.54% <100.00%> (-0.26%)	⬇️

[bhancock8]

The logic all looks good. The only thing that bugs me is having predict() and score() being strictly pass-through methods on LabelModel. The duplicated passing of arguments and documentation invites staleness. The documentation demonstrating usage should be covered by unit tests anyway. I'd vote we just let the LabelModel inherit those methods from its parent.

[bhancock8]

Discussed IRL. Decision to leave the methods on LabelModel to keep the Examples section for the documentation.