docstr: A method that returns an encoded key that represents
  the current batch and param location.
  Args:
      param_loc (int): bucket location sent by the trainer
          containing the gradient
docstr_tok: ["A","method","that","returns","an","encoded","key","that","represents","the","current","batch","and","param","location",".","Args",":","param_loc","(","int",")",":","bucket","location","sent","by","the","trainer","containing","the","gradient"]
code: def param_key(self, param_loc):
      r"""
      A method that returns an encoded key that represents
      the current batch and param location.
      Args:
          param_loc (int): bucket location sent by the trainer
              containing the gradient
      """
      return f"{self.batch_number},{param_loc}"
  
imports: import functools
  import threading
  import time
  from abc import ABC, abstractmethod
  import torch
  import torch.distributed.rpc
  from metrics.MetricsLogger import MetricsLogger
  from utils import sparse_rpc_format_to_tensor, sparse_tensor_to_rpc_format
label_desc: Use image node_id 3 for calling the AverageParameterServer obj's underlying member method code with example usage: obj.param_key(param_loc) and returns: self, str, param_loc