code: def base64url_decode(input):
      
      rem = len(input) % 4
  
      if rem > 0:
          input += b'=' * (4 - rem)
  
      return base64.urlsafe_b64decode(input)
docs: Helper method to base64url_decode a string.
  
      Args:
          input (str): A base64url_encoded string to decode.