add SweepablePipeline

[LittleLittleCloud]

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This PR implements SweepablePipeline according to spec in #6218

The new SweepablePipeline will replace MultiModelPipeline and SweepableEstimatorPipeline and would be helpful in constructing complex sweepable pipeline as well as facilitating future optimization over sweeping method

[codecov]

Codecov Report

Merging #6222 (bda4fcd) into main (4715e61) will increase coverage by 0.03%.
The diff coverage is 87.87%.

@@            Coverage Diff             @@
##             main    #6222      +/-   ##
==========================================
+ Coverage   68.43%   68.46%   +0.03%     
==========================================
  Files        1139     1140       +1     
  Lines      244596   244801     +205     
  Branches    25390    25435      +45     
==========================================
+ Hits       167384   167602     +218     
+ Misses      70577    70563      -14     
- Partials     6635     6636       +1     

Flag	Coverage Δ
Debug	68.46% <87.87%> (+0.03%)	⬆️
production	62.93% <ø> (+0.04%)	⬆️
test	89.01% <87.87%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
...icrosoft.ML.AutoML.Tests/SweepablePipelineTests.cs	87.87% <87.87%> (ø)
...c/Microsoft.ML.FastTree/Utils/ThreadTaskManager.cs	79.48% <0.00%> (-20.52%)	⬇️
...soft.ML.Data/DataLoadSave/Text/TextLoaderCursor.cs	89.77% <0.00%> (-0.48%)	⬇️
...ML.Transforms/Text/StopWordsRemovingTransformer.cs	86.38% <0.00%> (-0.15%)	⬇️
test/Microsoft.ML.AutoML.Tests/AutoFitTests.cs	89.04% <0.00%> (ø)
src/Microsoft.ML.TorchSharp/TorchSharpCatalog.cs	100.00% <0.00%> (ø)
...crosoft.ML.StandardTrainers/Standard/SdcaBinary.cs	88.82% <0.00%> (ø)
src/Microsoft.ML.Maml/MAML.cs	25.38% <0.00%> (+1.01%)	⬆️
...osoft.ML.KMeansClustering/KMeansPlusPlusTrainer.cs	92.32% <0.00%> (+1.24%)	⬆️
src/Microsoft.ML.FastTree/Training/StepSearch.cs	62.37% <0.00%> (+4.95%)	⬆️
... and 1 more

[michaelgsharp]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 2 pipeline(s).

[michaelgsharp]

Avoid this when possible.

[michaelgsharp]