Migrate the trainer to Lightning

[zhiqwang]

Closes #34

[codecov]

Codecov Report

Merging #43 (96f5800) into master (378a5c9) will increase coverage by 7.58%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   70.77%   78.35%   +7.58%     
==========================================
  Files           8        8              
  Lines         609      619      +10     
==========================================
+ Hits          431      485      +54     
+ Misses        178      134      -44     

Flag	Coverage Δ
unittests	78.35% <100.00%> (+7.58%)	⬆️

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Impacted Files	Coverage Δ
test/dataset_utils.py	100.00% <100.00%> (+100.00%)	⬆️
test/test_engine.py	97.72% <100.00%> (+0.66%)	⬆️

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