docstr: The number of instances processed by each one of the current forest members.
  
  Each time a concept drift is detected, the count corresponding to the affected tree is
  reset.
code: def instances_per_tree(self) -> collections.Counter:
      """The number of instances processed by each one of the current forest members.
  
      Each time a concept drift is detected, the count corresponding to the affected tree is
      reset.
      """
      return self._sample_counter
  
imports: from __future__ import annotations
  import abc
  import collections
  import math
  import random
  import sys
  from river import base, drift, metrics, tree, utils
  from river.tree.nodes.et_nodes import ETLeafAdaptive, ETLeafMean, ETLeafModel
  from river.tree.splitter.random_splitter import RegRandomSplitter
label_desc: Use image node_id 17 for calling the ExtraTrees obj's underlying member method code with example usage: obj.instances_per_tree() and returns: self