Added IDisposable to OnnxTransformer and fixed memory leaks

[harishsk]

Fixes #5342
A temp file was being created when the Onnx model was being loaded from within an ML.NET model. This file wasn't being deleted when the session exited because OnnxTransformer didn't support IDisposable. (Also added Dispose calls to OnnxTransformer objects in the tests)

Thanks to the reported bug, this also fixes memory leaks that we would see from OnnxRuntime.dll because the native memory associated with InferenceSession wasn't freed.

[codecov]

Codecov Report

Merging #5348 into master will increase coverage by 0.00%.
The diff coverage is 68.83%.

@@           Coverage Diff           @@
##           master    #5348   +/-   ##
=======================================
  Coverage   74.04%   74.04%           
=======================================
  Files        1019     1019           
  Lines      189949   190005   +56     
  Branches    20429    20455   +26     
=======================================
+ Hits       140651   140696   +45     
+ Misses      43786    43779    -7     
- Partials     5512     5530   +18     

Flag	Coverage Δ
#Debug	74.04% <68.83%> (+<0.01%)	⬆️
#production	69.85% <71.42%> (+<0.01%)	⬆️
#test	87.64% <68.57%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
test/Microsoft.ML.Tests/OnnxConversionTest.cs	96.17% <36.36%> (-0.52%)	⬇️
src/Microsoft.ML.OnnxTransformer/OnnxTransform.cs	91.60% <66.66%> (-0.38%)	⬇️
...osoft.ML.OnnxTransformerTest/OnnxTransformTests.cs	95.33% <74.13%> (-2.34%)	⬇️
src/Microsoft.ML.OnnxTransformer/OnnxUtils.cs	86.28% <100.00%> (-0.13%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	73.88% <100.00%> (+0.03%)	⬆️
...L.AutoML/TrainerExtensions/TrainerExtensionUtil.cs	84.71% <0.00%> (-1.66%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.53% <0.00%> (-0.15%)	⬇️
...rosoft.ML.Data/Scorers/PredictedLabelScorerBase.cs	81.95% <0.00%> (+0.37%)	⬆️
...Microsoft.ML.Data/Scorers/PredictionTransformer.cs	91.00% <0.00%> (+0.50%)	⬆️
src/Microsoft.ML.Featurizers/TimeSeriesImputer.cs	88.37% <0.00%> (+1.22%)	⬆️
... and 2 more