Using default OnnxSequenceType attribute throws meaningless exception

[eerhardt]

Pull the code at this commit and run the application - https://github.com/sethjuarez/SeeSharp/tree/b6a7e8a0094b99af05bc72528a2c6e306abbf7cd

The code of interest is:

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


        [ColumnName("loss")]
        [OnnxSequenceType]
        public List<float> Loss;
    }

You get the following error:

Unhandled Exception: System.ArgumentNullException: Value cannot be null.
   at System.RuntimeType.MakeGenericType(Type[] instantiation)
   at Microsoft.ML.Transforms.Onnx.OnnxSequenceTypeAttribute.Register()
   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 70
   at SeeSharp.Program.Main(String[] args) in /Users/eerhardt/git/SeeSharp/SeeSharp/Program.cs:line 39

It looks like the code here:

machinelearning/src/Microsoft.ML.OnnxTransformer/OnnxSequenceType.cs

Lines 95 to 100 in 0c0789f

	public override void Register()
	{
	var enumerableType = typeof(IEnumerable<>);
	var type = enumerableType.MakeGenericType(_elemType);
	DataViewTypeManager.Register(new OnnxSequenceType(_elemType), type, new[] { this });
	}

Is expecting _elemType to never be null. However, that is exactly what happens when you use the default attribute:

machinelearning/src/Microsoft.ML.OnnxTransformer/OnnxSequenceType.cs

Lines 57 to 64 in 0c0789f

	private Type _elemType;
	/// <summary>
	/// Create a sequence type.
	/// </summary>
	public OnnxSequenceTypeAttribute()
	{
	}

I don't think having a default constructor is right with this attribute - you can't use this sequence attribute without specifying an element type.

/cc @wschin