Convert LdaEngine to a SafeHandle

[sharwell]

This pull request follows the test failure observed in https://dev.azure.com/dnceng/public/_build/results?buildId=449759&view=ms.vss-test-web.build-test-results-tab&runId=14347330&resultId=100770&paneView=debug. Converting this object to a SafeHandle has two primary advantages:

1. The object will not be disposed while there is an ongoing native call
2. Misuse of the object by trying to pass an invalid instance to a native call will throw an exception at the point of the call

[codemzs]

[codecov]

Codecov Report

Merging #4538 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4538      +/-   ##
==========================================
+ Coverage   75.12%   75.12%   +<.01%     
==========================================
  Files         908      908              
  Lines      160214   160220       +6     
  Branches    17250    17250              
==========================================
+ Hits       120355   120365      +10     
+ Misses      35045    35039       -6     
- Partials     4814     4816       +2

Flag	Coverage Δ
#Debug	75.12% <100%> (ø)	⬆️
#production	70.52% <100%> (ø)	⬆️
#test	90.31% <ø> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/Microsoft.ML.Transforms/Text/LdaSingleBox.cs	63.63% <100%> (+1.13%)	⬆️
....ML.AutoML/PipelineSuggesters/PipelineSuggester.cs	79.83% <0%> (-3.37%)	⬇️
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	59.6% <0%> (-2.65%)	⬇️
...soft.ML.TestFramework/DataPipe/TestDataPipeBase.cs	76.08% <0%> (-0.4%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.26% <0%> (+0.15%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/ISweeper.cs	67.08% <0%> (+7.59%)	⬆️
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	100% <0%> (+20.51%)	⬆️

[sharwell]

With changes to the native library now merged (#4551), I reduced this pull request to the minimum necessary to stabilize the use of this particular handle. It specifically fixes cases where this exception appears in test runs:

  Error Message:
   System.InvalidOperationException : Splitter/consolidator worker encountered exception while consuming source data
---- System.AccessViolationException : Attempted to read or write protected memory. This is often an indication that other memory is corrupt.
  Stack Trace:
     at Microsoft.ML.Data.DataViewUtils.Splitter.Batch.SetAll(OutPipe[] pipes) in D:\a\1\s\src\Microsoft.ML.Data\Data\DataViewUtils.cs:line 832
   at Microsoft.ML.Data.DataViewUtils.Splitter.Cursor.MoveNextCore() in D:\a\1\s\src\Microsoft.ML.Data\Data\DataViewUtils.cs:line 1102
   at Microsoft.ML.Data.RootCursorBase.MoveNext() in D:\a\1\s\src\Microsoft.ML.Core\Data\RootCursorBase.cs:line 65
   at Microsoft.ML.RunTests.TestDataPipeNoBaseline.TestLDATransform() in D:\a\1\s\test\Microsoft.ML.TestFramework\DataPipe\TestDataPipe.cs:line 1335
----- Inner Stack Trace -----
   at Microsoft.ML.TextAnalytics.LdaInterface.InitializeBeforeTest(LdaEngine engine)
   at Microsoft.ML.Transforms.Text.LatentDirichletAllocationTransformer.LdaState.Output(VBuffer`1& src, VBuffer`1& dst, Int32 numBurninIter, Boolean reset) in D:\a\1\s\src\Microsoft.ML.Transforms\Text\LdaTransform.cs:line 470
   at Microsoft.ML.Transforms.Text.LatentDirichletAllocationTransformer.Mapper.<>c__DisplayClass5_0.<GetTopic>b__0(VBuffer`1& dst) in D:\a\1\s\src\Microsoft.ML.Transforms\Text\LdaTransform.cs:line 612
   at Microsoft.ML.Data.DataViewUtils.Splitter.InPipe.Impl`1.Fill() in D:\a\1\s\src\Microsoft.ML.Data\Data\DataViewUtils.cs:line 723
   at Microsoft.ML.Data.DataViewUtils.Splitter.<>c__DisplayClass5_1.<ConsolidateCore>b__2() in D:\a\1\s\src\Microsoft.ML.Data\Data\DataViewUtils.cs:line 415