Matrix factorization should work in x86

[artidoro]

Matrix factorization fails in x86 (MatrixFactorizationSimpleTrainAndPredict test was disabled as part of #1432).

The reason is that the MFModel struct is architecture dependent:

machinelearning/src/Microsoft.ML.Recommender/SafeTrainingAndModelBuffer.cs

Lines 67 to 80 in eae7695

	[StructLayout(LayoutKind.Explicit)]
	private unsafe struct MFModel
	{
	[FieldOffset(0)]
	public int M;
	[FieldOffset(4)]
	public int N;
	[FieldOffset(8)]
	public int K;
	[FieldOffset(16)]
	public float* P;
	[FieldOffset(24)]
	public float* Q;
	}

The struct matches the output data structure produced by c++ code in the libmf library.

See the following comment for how this issue should be solved using P/Invoke instead: #1432 (comment)

[eerhardt]

Looks like users hit this as well:

https://twitter.com/diederikkrols/status/1111013594978795522

When you try using Matrix Factorization in an x86 application, you get an exception:

System.ExecutionEngineException: Exception of type 'System.ExecutionEngineException' was thrown.
>	System.Private.CoreLib.dll!System.Runtime.InteropServices.Marshal.Copy(System.IntPtr source, float[] destination, int startIndex, int length) Line 363	C#
 	Microsoft.ML.Recommender.dll!Microsoft.ML.Recommender.Internal.SafeTrainingAndModelBuffer.Get(out int m, out int n, out int k, out float[] p, out float[] q) Line 392	C#
 	Microsoft.ML.Recommender.dll!Microsoft.ML.Trainers.Recommender.MatrixFactorizationModelParameters.MatrixFactorizationModelParameters(Microsoft.ML.Runtime.IHostEnvironment env, Microsoft.ML.Recommender.Internal.SafeTrainingAndModelBuffer buffer, Microsoft.ML.Data.KeyDataViewType matrixColumnIndexType, Microsoft.ML.Data.KeyDataViewType matrixRowIndexType) Line 102	C#
 	Microsoft.ML.Recommender.dll!Microsoft.ML.Trainers.MatrixFactorizationTrainer.TrainCore(Microsoft.ML.Runtime.IChannel ch, Microsoft.ML.Data.RoleMappedData data, Microsoft.ML.Data.RoleMappedData validData) Line 459	C#
 	Microsoft.ML.Recommender.dll!Microsoft.ML.Trainers.MatrixFactorizationTrainer.Fit(Microsoft.ML.IDataView trainData, Microsoft.ML.IDataView validationData) Line 510	C#
 	Microsoft.ML.Recommender.dll!Microsoft.ML.Trainers.MatrixFactorizationTrainer.Fit(Microsoft.ML.IDataView input) Line 520	C#
 	Microsoft.ML.Data.dll!Microsoft.ML.Data.EstimatorChain<Microsoft.ML.Transforms.KeyToValueMappingTransformer>.Fit(Microsoft.ML.IDataView input)	Unknown

cc @wschin @XamlBrewer