Refactor the import and support checkpoint optimizer and lr_scheduler state

[senwu]

Description of the problems or issues

The current Emmental cannot checkpoint optimizer and lr_scheduler state.

Description of the proposed changes

Support checkpoint optimizer and lr_scheduler state

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[bot]]

Codecov Report

Merging #93 (22231cb) into master (06657d8) will decrease coverage by 1.85%. The diff coverage is 71.27%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   88.09%   86.24%   -1.86%     
==========================================
  Files          38       39       +1     
  Lines        1798     1883      +85     
  Branches      389      406      +17     
==========================================
+ Hits         1584     1624      +40     
- Misses        108      147      +39     
- Partials      106      112       +6     

Flag	Coverage Δ
unittests	86.24% <71.27%> (-1.86%)	:arrow_down:

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

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)
src/emmental/logging/logging_manager.py	86.53% <30.00%> (-13.47%)	:arrow_down:
src/emmental/learner.py	70.27% <50.00%> (-5.80%)	:arrow_down:
src/emmental/utils/seed.py	82.60% <82.60%> (ø)
src/emmental/model.py	82.58% <93.10%> (+0.15%)	:arrow_up:
src/emmental/__init__.py	100.00% <100.00%> (ø)
src/emmental/_version.py	100.00% <100.00%> (ø)
src/emmental/logging/checkpointer.py	88.57% <100.00%> (+0.33%)	:arrow_up:
src/emmental/meta.py	79.31% <100.00%> (-0.69%)	:arrow_down:
src/emmental/metrics/accuracy_f1.py	100.00% <100.00%> (ø)
... and 8 more