code: def __call__(self, step):
      arg1 = step**-0.5
      arg2 = step * (self.warmup_steps**-1.5)
      return self.learning_rate_start * tf.math.minimum(arg1, arg2)
  
imports: import os
  import pickle
  from typing import Callable
  from typing import Dict, Union
  import numpy
  import tensorflow
  from headliner.model.bert_model import Transformer, create_masks
  from headliner.model.summarizer import Summarizer
  from headliner.preprocessing.bert_vectorizer import BertVectorizer
  from headliner.preprocessing.preprocessor import Preprocessor
  from headliner.utils.logger import get_logger
label_desc: Use image node_id 2 for calling the CustomSchedule obj's underlying member method code with example usage: obj.__call__(step) and returns: unknown