class_docstr: Optimizer interface.
  
  Every optimizer inherits from this base interface.
  
  Parameters
  ----------
  lr
  
  Attributes
  ----------
  learning_rate : float
      Returns the current learning rate value.
code: def learning_rate(self) -> float:
      return self.lr.get(self.n_iterations)
  
imports: from __future__ import annotations
  import abc
  import typing
  import numpy
  from river import base, optim, utils
label_desc: Use image node_id 3 for calling the Optimizer obj's underlying member method code with example usage: obj.learning_rate() and returns: self