code: def learn_one(self, x: dict, y: base.typing.RegTarget, w: int = 1):
      self.mean.update(y, w)
  
imports: from __future__ import annotations
  import collections
  import functools
  import io
  import math
  import typing
  from river import anomaly, base, drift, linear_model, stats
  from river.tree import split_criterion
  from river.tree import splitter
  from .base import HoeffdingRule
label_desc: Use image node_id 2 for calling the MeanRegressor obj's underlying member method code with example usage: obj.learn_one(x, y, w) without return types