Using OnnxSequenceType and ColumnName attributes together doesn't work

[eerhardt]

If you have both OnnxSequenceType and ColumnName attributes on a property, ML.NET gets confused and throws an exception.

For example, if you pull the following code:

https://github.com/sethjuarez/SeeSharp/tree/c864892252880d4ef87006008823dee3b29adf9c

And change the ImagePrediction class to be:

    public class ImagePrediction
    {
        [ColumnName("classLabel")]
        [VectorType]
        public string[] Prediction;

        [ColumnName("loss")]
        [OnnxSequenceType(typeof(IDictionary<string, float>))]
        public IEnumerable<IDictionary<string, float>> Loss;
    }

(You will also need to change 1 place in the Main method to change from output.loss.FirstOrDefault(); to output.Loss.FirstOrDefault(); from the rename.)

You get an error:

Unhandled Exception: System.ArgumentOutOfRangeException: Could not determine an IDataView type for member Loss
Parameter name: rawType
   at Microsoft.ML.Data.InternalSchemaDefinition.GetVectorAndItemType(String name, Type rawType, IEnumerable`1 attributes, Boolean& isVector, Type& itemType)
   at Microsoft.ML.Data.SchemaDefinition.Create(Type userType, Direction direction)
   at Microsoft.ML.Data.TypedCursorable`1.Create(IHostEnvironment env, IDataView data, Boolean ignoreMissingColumns, SchemaDefinition schemaDefinition)
   at Microsoft.ML.PredictionEngineBase`2.PredictionEngineCore(IHostEnvironment env, InputRow`1 inputRow, IRowToRowMapper mapper, Boolean ignoreMissingColumns, SchemaDefinition outputSchemaDefinition, Action& disposer, IRowReadableAs`1& outputRow)
   at Microsoft.ML.PredictionEngineBase`2..ctor(IHostEnvironment env, ITransformer transformer, Boolean ignoreMissingColumns, SchemaDefinition inputSchemaDefinition, SchemaDefinition outputSchemaDefinition)
   at Microsoft.ML.PredictionEngineExtensions.CreatePredictionEngine[TSrc,TDst](ITransformer transformer, IHostEnvironment env, Boolean ignoreMissingColumns, SchemaDefinition inputSchemaDefinition, SchemaDefinition outputSchemaDefinition)
   at SeeSharp.Program.LoadModel(String onnxModelFilePath) in /Users/eerhardt/git/SeeSharp/SeeSharp/Program.cs:line 76
   at SeeSharp.Program.Main(String[] args) in /Users/eerhardt/git/SeeSharp/SeeSharp/Program.cs:line 39

This should work. The program works if you remove the ColumnName attribute. And fails with the above as soon as you add the ColumnName attribute.

/cc @wschin