Update Dataset type to support multilabel datasets

[PatrickLaflamme]

Currently, the Dataset abstract class only supports single-class datasets. This is because of the getY method defined here:

https://github.com/Kotlin/kotlindl/blob/master/dataset/src/jvmMain/kotlin/org/jetbrains/kotlinx/dl/dataset/Dataset.kt#L23

which only returns a single Float per index, instead of a FloatData.

[zaleslaw]

@PatrickLaflamme Could you give a case when it could be useful for KotlinDL? Did you try to do some cases?