static Optional<Integer> toInteger(Optional<String> integerText) {
        if ( ! integerText.isPresent()) return Optional.empty();
        return Optional.of(Integer.parseInt(integerText.get()));
    }
Returns the number of samples in the training set that matches this node
if this model does not contain this information (i.e if it is not an "ext" model).
searchlib/src/main/java/com/yahoo/searchlib/gbdt/TreeNode.java