Support online_eval to improve training performance

[senwu]

Description of the problems or issues

Support online_eval to improve training performance

Description of the proposed changes

Add online_eval argument

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 #89 (c5b748b) into master (1a35ab3) will decrease coverage by 0.28%.
The diff coverage is 73.68%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   87.73%   87.45%   -0.29%     
==========================================
  Files          38       38              
  Lines        1745     1753       +8     
  Branches      375      379       +4     
==========================================
+ Hits         1531     1533       +2     
- Misses        114      116       +2     
- Partials      100      104       +4     

Flag	Coverage Δ
unittests	87.45% <73.68%> (-0.29%)	⬇️

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

Impacted Files	Coverage Δ
src/emmental/meta.py	80.00% <ø> (ø)
src/emmental/model.py	80.06% <72.72%> (-1.32%)	⬇️
src/emmental/learner.py	76.06% <75.00%> (-0.30%)	⬇️
src/emmental/utils/parse_args.py	99.27% <100.00%> (+<0.01%)	⬆️