docs: def UploadUsers(self, hash_algorithm, hash_key, accounts):
      """Uploads multiple users to Gitkit server.
  
      Args:
        hash_algorithm: string, the hash algorithm.
        hash_key: array, raw key of the hash algorithm.
        accounts: list of GitkitUser.
  
      Returns:
        A dict of failed accounts. The key is the index of the 'accounts' list,
            starting from 0.
      """
      return self.rpc_helper.UploadAccount(hash_algorithm,
                                           base64.urlsafe_b64encode(hash_key),
                                           [GitkitUser.ToRequest(i) for i in accounts])