SVMLightLoader Fails above 128 dense rows

### System information

- **OS version/distro**: macOS 10.14.6
- **.NET Version (eg., dotnet --info)**: 5.0.101

### Issue

SVMLightLoader dies if when you load >128 dense rows.

When the feature [column sparsity is >0.25](https://github.com/dotnet/machinelearning/blob/cf7dbffa50f7bd65172bfcb11537207a81250994/src/Microsoft.ML.Core/Utilities/VBufferUtils.cs#L21), internally the column is represented in sparse format, else dense. SVMLightLoader works if either the column is sparse (many missing values), or if the number of rows is < 128.

#### Error
Fails with one of three errors: _(dataset dependent)_
* > System.InvalidOperationException: Duplicate keys found in dataset
* > System.ArgumentException: Destination is too short. (Parameter 'destination')
* > System.IndexOutOfRangeException: Index was outside the bounds of the array.

Stack trace:

> ```
> Unhandled exception. System.InvalidOperationException: Splitter/consolidator worker encountered exception while consuming source data
> ---> System.InvalidOperationException: Duplicate keys found in dataset
> at Microsoft.ML.Data.SvmLightLoader.OutputMapper.MapCore(VBuffer`1& keys, VBuffer`1& values, Output output)
> at Microsoft.ML.Data.SvmLightLoader.OutputMapper.Map(IntermediateOut intermediate, Output output)
> at Microsoft.ML.Transforms.CustomMappingTransformer`2.Mapper.<>c__DisplayClass5_0.b__0()
> at Microsoft.ML.Transforms.CustomMappingTransformer`2.Mapper.<>c__DisplayClass6_0`1.b__0(T& dst)
> at Microsoft.ML.Data.DataViewUtils.Splitter.InPipe.Impl`1.Fill()
> at Microsoft.ML.Data.DataViewUtils.Splitter.<>c__DisplayClass7_1.b__2()
> --- End of inner exception stack trace ---
> at Microsoft.ML.Data.DataViewUtils.Splitter.Batch.SetAll(OutPipe[] pipes)
> at Microsoft.ML.Data.DataViewUtils.Splitter.Cursor.MoveNextCore()
> at Microsoft.ML.Data.RootCursorBase.MoveNext()
> at Microsoft.ML.Data.SynchronizedCursorBase.MoveNext()
> at SVMLightLoaderTest.Program.PrintData(IDataView svmData) in /Users/justinormont/Projects/SVMLightLoaderTest/SVMLightLoaderTest/Program.cs:line 121
> at SVMLightLoaderTest.Program.Main() in /Users/justinormont/Projects/SVMLightLoaderTest/SVMLightLoaderTest/Program.cs:line 45
> ```

Points to:
https://github.com/dotnet/machinelearning/blob/5dbfd8acac0bf798957eea122f1413209cdf07dc/src/Microsoft.ML.Transforms/SvmLight/SvmLightLoader.cs#L364-L388

Side note: It looks like Visual Studio on MacOS is not loading the symbols (or source) for ML․NET.

### Source code / logs

Repro:
* .NET Fiddle -- https://dotnetfiddle.net/WbKlzS
* Visual Studio Solution: [SVMLightLoaderTest.zip](https://github.com/dotnet/machinelearning/files/5730051/SVMLightLoaderTest.zip)

Bug exists in ML․NET v1.5.0 to v.1.5.4 (current). SvmLightLoader was added in v1.5.0.

## Background

I was attempting to run AutoML․NET on a [SVM Light dataset](https://www.sysnet.ucsd.edu/projects/url/) ([download](https://www.sysnet.ucsd.edu/projects/url/url_svmlight.tar.gz)) using the CLI. But we lack SVM Light support in AutoML․NET, so I was attempting to convert the SVM Light file to a sparse TSV. The goal was to have AutoML․NET read the converted sparse TSV file, but the conversion failed.

Using MAML in v1.5.4: _(fails)_
`dotnet ./bin/AnyCPU.Release/Microsoft.ML.Console/netcoreapp2.1/MML.dll SaveData data=Day0.svm loader=SvmLightLoader{} xf=SelectColumns{keep=Label keep=Features} saver=Text{schema=- dense=-} dout=Day0.tsv`

This fails with the above errors, as the current SvmLightLoader fails.

Using TLC's MAML: _(works)_
`maml.exe SaveData data=Day0.svm loader=SvmLightLoader{} xf=KeepColumns{col=Label col=Features} saver=Text{schema=- dense=-} dout=Day0.tsv`

The old internal version of ML․NET (TLC) works properly in reading the SVM Light format and writing a TSV. The implies there was a bug introduced when we released SvmLightLoader with v1.5.0 of ML․NET.