Fixed PFI bug with transformer chains

[michaelgsharp]

Fixes #6027

The new pfi APi we added to make it easier for users has an issue with handling Transformer chains incorrectly. This PR fixes that and adds a test using Transformer chains.

[codecov]

Codecov Report

Merging #6085 (14f569d) into main (3b5e6a0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #6085   +/-   ##
=======================================
  Coverage   68.60%   68.60%           
=======================================
  Files        1146     1146           
  Lines      245451   245490   +39     
  Branches    25638    25640    +2     
=======================================
+ Hits       168380   168428   +48     
+ Misses      70375    70373    -2     
+ Partials     6696     6689    -7     

Flag	Coverage Δ
Debug	68.60% <100.00%> (+<0.01%)	⬆️
production	63.40% <100.00%> (+<0.01%)	⬆️
test	88.68% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...ansforms/PermutationFeatureImportanceExtensions.cs	95.72% <100.00%> (+3.41%)	⬆️
...soft.ML.Tests/PermutationFeatureImportanceTests.cs	100.00% <100.00%> (ø)
...rosoft.ML.AutoML/ColumnInference/TextFileSample.cs	59.60% <0.00%> (-5.97%)	⬇️
test/Microsoft.ML.AutoML.Tests/AutoFitTests.cs	87.07% <0.00%> (-1.97%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.38% <0.00%> (+0.14%)	⬆️
...StandardTrainers/Standard/LinearModelParameters.cs	66.07% <0.00%> (+0.25%)	⬆️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	90.41% <0.00%> (+0.63%)	⬆️
src/Microsoft.ML.AutoML/Sweepers/Parameters.cs	85.59% <0.00%> (+0.84%)	⬆️
src/Microsoft.ML.Maml/MAML.cs	26.90% <0.00%> (+2.53%)	⬆️
... and 1 more