WIP Safe pointers

[sharwell]

Locally I converted most owned native objects to std::unique_ptr and std::shared_ptr. This pull request represents a relatively easy-to-review subset of the changes with minimal overall fan-out in the code.

• Pass by reference when null is not expected (submitted separately as Pass by reference when null is not expected #4546)
• Use std::unique_ptr for objects in LdaEngine (submitted separately as Use std::unique_ptr for objects in LdaEngine #4547)
• Use std::unique_ptr for samplers_ and likelihood_in_iter_

[sharwell]

📝 This block was responsible for some of the crashes observed in CI, and especially the crashes prior to 0cbfa54. The destructor is missing a null check for samplers_.

[codecov]

Codecov Report

Merging #4544 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4544      +/-   ##
==========================================
- Coverage    75.1%    75.1%   -0.01%     
==========================================
  Files         908      908              
  Lines      160260   160260              
  Branches    17259    17259              
==========================================
- Hits       120369   120357      -12     
- Misses      35070    35086      +16     
+ Partials     4821     4817       -4

Flag	Coverage Δ
#Debug	75.1% <ø> (-0.01%)	⬇️
#production	70.5% <ø> (-0.01%)	⬇️
#test	90.26% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
src/Microsoft.ML.AutoML/Sweepers/ISweeper.cs	59.49% <0%> (-7.6%)	⬇️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	77.96% <0%> (-5.09%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	84.9% <0%> (-0.21%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	84.44% <0%> (-0.16%)	⬇️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	64.9% <0%> (+5.29%)	⬆️

[sharwell]

Superseded by #4551.