class_docstr: The learning rate schedule callback decreases the learning rate by a
  predefined factor after each of the provided milestones (after x seconds
  during training).
code: def on_train_start(self, trainer: Trainer) -> None:
      self.trainer = trainer
      self.lr = trainer.learning_rate
      self.milestone_index = 0
  
file_path: gluonts/src/gluonts/nursery/tsbench/src/tsbench/gluonts/callbacks/learning_rate.py
imports: from typing import cast, List, Optional
  from mxnet.gluon import nn
  from mxnet.gluon.trainer import Trainer
  from .base import Callback
label_desc: Use image node_id 2 for calling the LearningRateScheduleCallback obj's underlying member method code with example usage: obj.on_train_start(trainer) without return types