make_tree: A utility function for generating random trees for test purposes.
Description
Build a depth depth tree by drawing random split variables
and split values from the Nxp matrix X. In leaf nodes a random
action is drawn from 1:d. (Minimum leaf size will be 1)
if (FALSE) {
depth <- 2
n <- 100
p <- 10
d <- 3
X <- matrix(rnorm(n * p), n, p)
Y <- matrix(rnorm(n * d), n, d)
tree <- make_tree(X, depth = depth, d = d)
pp <- predict_test_tree(tree, X)
}