OvaModelParameters is not strongly-typed

In the constructor of 'OvaModelParameters`,
```csharp
using TScalarPredictor = IPredictorProducing;
...
internal static OvaModelParameters Create(IHost host, OutputFormula outputFormula, TScalarPredictor[] predictors)
{
...
}
```
we remove the type information of predictors by forcing them to be `IPredictorProducing`. To make all multi-class classifiers' outputting strongly-typed models. We need to make `OvaModelParameters` strongly-typed.