docstr: A method that returns an encoded key that represents the current epoch and
  iteration index.
  Args:
      epoch (int): epoch index
      index (int): iteration index
docstr_tok: ["A","method","that","returns","an","encoded","key","that","represents","the","current","epoch","and","iteration","index",".","Args",":","epoch","(","int",")",":","epoch","index","index","(","int",")",":","iteration","index"]
code: def epoch_key(self, epoch, index):
      r"""
      A method that returns an encoded key that represents the current epoch and
      iteration index.
      Args:
          epoch (int): epoch index
          index (int): iteration index
      """
      return f"{epoch},{index}"
  
imports: import functools
  import time
  from abc import ABC, abstractmethod
  import torch
  from metrics.MetricsLogger import MetricsLogger
label_desc: Use image node_id 2 for calling the DdpTrainer obj's underlying member method code with example usage: obj.epoch_key(epoch, index) and returns: epoch, str, index