class_docstr: Class used to monitor and update the gradient/hessian information in Stochastic Gradient
  Trees.
  
  Represents the aggregated gradient/hessian data in a node (global node statistics), category,
  or numerical feature's discretized bin.
code: def __init__(self):
      self.g_var = Var()
      self.h_var = Var()
      self.gh_cov = Cov()
  
imports: from __future__ import annotations
  import collections
  import copy
  import dataclasses
  import functools
  import math
  import sys
  import typing
  from river.base.typing import FeatureName
  from river.stats import Cov, Var
label_desc: Use image node_id 1 to create a new GradHessStats object  with example: obj = GradHessStats()