:: DeveloperApi ::
Internal Decision Tree node.
param: prediction Prediction this node would make if it were a leaf node
param: impurity Impurity measure at this node (for training data)
param: gain Information gain value.
Values < 0 indicate missing values; this quirk will be removed with future updates.
param: leftChild Left-hand child node
param: rightChild Right-hand child node
param: split Information about the test used to split to the left or right child.
Statistics aggregated from training data at this node, used to compute prediction, impurity,
For classification, the array of class counts must be normalized to a probability distribution.