Disable CS0649 in OnnxConversionTest

[sharwell]

Fixes an IDE0051 warning (Private member is unused).

[codemzs]

[codecov]

Codecov Report

Merging #4531 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #4531      +/-   ##
==========================================
+ Coverage    75.1%   75.11%   +<.01%     
==========================================
  Files         908      908              
  Lines      160260   160253       -7     
  Branches    17259    17259              
==========================================
+ Hits       120369   120376       +7     
+ Misses      35070    35061       -9     
+ Partials     4821     4816       -5

Flag	Coverage Δ
#Debug	75.11% <ø> (ø)	⬆️
#production	70.51% <ø> (ø)	⬆️
#test	90.28% <ø> (+0.01%)	⬆️

Impacted Files	Coverage Δ
test/Microsoft.ML.Tests/OnnxConversionTest.cs	95.85% <ø> (+0.62%)	⬆️
...L.AutoML/TrainerExtensions/TrainerExtensionUtil.cs	85.15% <0%> (-1.75%)	⬇️
...soft.ML.Transforms/Text/WordEmbeddingsExtractor.cs	86.39% <0%> (-1.14%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.1% <0%> (-0.16%)	⬇️
src/Microsoft.ML.Transforms/Text/LdaTransform.cs	84.44% <0%> (-0.16%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	85.31% <0%> (+0.2%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	83.89% <0%> (+0.84%)	⬆️
src/Microsoft.ML.Maml/MAML.cs	26.21% <0%> (+1.45%)	⬆️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	62.25% <0%> (+2.64%)	⬆️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	86.55% <0%> (+6.72%)	⬆️