The Iris classification sample way of reusing the "Label" column is confusing

As mentioned by this ML.NET user, the way the [Iris Multiclassification sample](https://github.com/dotnet/machinelearning-samples/tree/master/samples/csharp/getting-started/MulticlassClassification_Iris) reuses the "Label" column is confusing:

https://github.com/dotnet/machinelearning/issues/5264#issuecomment-652487403.

The problem is that in the sample, we have this:
```C#
.Append(mlContext.Transforms.Conversion.MapKeyToValue(outputColumnName: nameof(IrisData.Label) , inputColumnName: "KeyColumn"));

```
Which the user interpreted that it was mapping the result of the prediction to the `Label` field.

What that step is doing is to actually map the key version of `Label` back to the original value of `Label` itself. Still, I don't know why this step is necessary, since the Label column itself is never modified (the MapValueToKey used at the beginning of the pipeline is actually mapping the Label column into a new KeyColumn, so no need to map back those values).

The lack of a `MapKeyToValue` that actually mapped back the values of "PredictedLabel" was what confused the user.

Furthermore, I also find strange that later when [consuming the model](https://github.com/dotnet/machinelearning-samples/tree/master/samples/csharp/getting-started/MulticlassClassification_Iris#4-consume-model) the approach to get the values for the key "PredictedLabel" column is to actually get the annotations. Which is somewhat a weird approach, since I'd find using a `MapKeyToValue` to be more user-friendly.

I guess a closer look to this is needed to determine if there's a reason behind those decisions. But at first look it seems that they're unnecessary, and it would be less confusing for users if we get rid of them. Thanks 😄

samples/csharp/getting-started/MulticlassClassification_Iris から始め、トレーニング パイプラインでの MapValueToKey と MapKeyToValue の使用方法を確認します。次に、リンクされている「Consume model」セクションを読んで、アノテーション ベースのアクセスと提案されている予測マッピングを比較します。必要な手順を確認し、ラベルと予測ラベルの扱いが明確かつ一貫するようにサンプルを更新します。

索引モデルが issue の本文から書いたものです。