code: def rmse(df, w):
      """Root mean squared error
  
      Parameters
      ----------
      df: Cross-validation results dataframe.
      w: Aggregation window size.
  
      Returns
      -------
      Dataframe with columns horizon and rmse.
      """
      res = mse(df, w)
      res["mse"] = np.sqrt(res["mse"])
      res.rename({"mse": "rmse"}, axis="columns", inplace=True)
      return res
  
imports: from __future__ import absolute_import, division, print_function
  import logging
  from tqdm.auto import tqdm
  from copy import deepcopy
  import concurrent.futures
  import numpy
  import pandas
label_desc: Use image node_id 9 for calling a global function with example usage: rmse(df, w) and returns: res