Add require_prob_for_eval and require_pred_for_eval to optimize score function

[senwu]

Description of the problems or issues

During training and inference probs or preds are not always necessary and might cause cpu memory waste.

Description of the proposed changes

Add require_prob_for_eval and require_pred_for_eval in EmmentalTask to let user specify whether those two are necessary.

Test plan

Test when different options.

Checklist

• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated the CHANGELOG.rst accordingly.

[codecov]

Codecov Report

Merging #92 (c34b190) into master (ae511cb) will increase coverage by 0.19%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   87.90%   88.09%   +0.19%     
==========================================
  Files          38       38              
  Lines        1777     1798      +21     
  Branches      385      389       +4     
==========================================
+ Hits         1562     1584      +22     
  Misses        108      108              
+ Partials      107      106       -1     

Flag	Coverage Δ
unittests	88.09% <94.11%> (+0.19%)	⬆️

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

Impacted Files	Coverage Δ
src/emmental/learner.py	76.06% <ø> (ø)
src/emmental/model.py	82.42% <93.33%> (+1.39%)	⬆️
src/emmental/scorer.py	83.87% <100.00%> (ø)
src/emmental/task.py	84.37% <100.00%> (+1.04%)	⬆️