How to append estimator after a LightGBM trainer with early stopping

Version: 0.11

When using early stopping (which is fantastic) with LightGBM you have to use:

```C#
var trainer = ctx.MulticlassClassification.Trainers.LightGbm(options);
var model = trainer.Fit(trainEncodedDv, testEncodedDv);
```

However, I need to append other estimators after the trainer. Like:

```C#
trainer = trainer.Append(ctx.Transforms.Conversion.MapKeyToValue(DefaultColumnNames.PredictedLabel))
.Append(ctx.Transforms.Conversion.MapKeyToValue((nameof(MlNetPrediction.OriginalLabelData), DefaultColumnNames.Label)));
```

Which of course I cannot do since then I cannot call the *two*-arity `fit` method anymore.

How can I solve this issue?

Btw: I think early stopping is absolutely crucial for any machine learning and IMO all iterative algorithms should support it.