Class defining state of the tree.

level_buffer: A tf.Tensor saves the last node value of the left child entered for the tree levels recorded in level_buffer_idx. level_buffer_idx: A tf.Tensor for the tree level index of the level_buffer. The tree level index starts from 0, i.e., level_buffer[0] when level_buffer_idx[0]==0 recorded the noise value for the most recent leaf node.
level_buffer A namedtuple alias for field number 0
level_buffer_idx A namedtuple alias for field number 1
value_generator_state State of a stateful ValueGenerator for tree node.