memory graph

[jsenellart]

better memory optimization using the computation graph, implement first vertical memory sharing

[codecov-io]

Codecov Report

Merging #303 into master will increase coverage by 0.34%.
The diff coverage is 93.33%.

@@            Coverage Diff            @@
##           master    #303      +/-   ##
=========================================
+ Coverage   77.15%   77.5%   +0.34%     
=========================================
  Files          66      67       +1     
  Lines        5270    5356      +86     
=========================================
+ Hits         4066    4151      +85     
- Misses       1204    1205       +1

Impacted Files	Coverage Δ
onmt/utils/ExtendedCmdLine.lua	70.73% <100%> (+1.98%)	⬆️
onmt/utils/init.lua	100% <100%> (ø)	⬆️
onmt/utils/MemoryGraph.lua	100% <100%> (ø)
onmt/Model.lua	79.31% <60%> (-4.03%)	⬇️
onmt/utils/Features.lua	98.38% <0%> (+1.61%)	⬆️

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[jsenellart]

temporary withdrawing to extend to non-nngraph models