Get Loss During Training for Visualization (Learning Curve Graph)

**Is your feature request related to a problem? Please describe.**
I need a way to visualize how my model is learning during training, which is a comparison between training loss and test loss.

**Describe the solution you'd like**
An event handler that enables the ability to extract loss during training.

**Describe alternatives you've considered**
Running the model for x epochs, evaluating the model, then retraining the model in a loop. This unfortunately does not work for all models, such as LightGbm that can't be retrained.

```csharp
var kfold = ctx.BinaryClassification.CrossValidate(training, estimator, param.kfold);
var bestModel = kfold.OrderByDescending(p => p.Metrics.Accuracy).Select(p => p.Model).First();

var testOutput = bestModel.Transform(test);

var metrics = ctx.BinaryClassification.Evaluate(testOutput);

// This code doesn't work as estimator is IEstimator and bestModel is ITransform. Not sure how you would do this...
estimator = bestModel;
```

**Additional context**
Ultimately, I am trying to analyze what the models I'm comparing are actually doing and so far I haven't found any documentation or any straightforward way to do it.