input: Write a python function to sort a DataFrame by its index in descending order.
  assert sort_dataframe_by_index(pd.DataFrame(data={'Value': [1, 2, 3, 4, 5]}, index=[5, 4, 3, 2, 1])).equals(pd.DataFrame(data={'Value': [1, 2, 3, 4, 5]}, index=[5, 4, 3, 2, 1]))
  assert sort_dataframe_by_index(pd.DataFrame(data={'Value': [10, 20, 30, 40, 50]}, index=[3, 1, 4, 5, 2])).equals(pd.DataFrame(data={'Value': [40, 30, 10, 50, 20]}, index=[5, 4, 3, 2, 1]))
  assert sort_dataframe_by_index(pd.DataFrame(data={'Value': [7, 8, 9]}, index=[3, 2, 1])).equals(pd.DataFrame(data={'Value': [9, 8, 7]}, index=[3, 2, 1]))
output: import pandas as pd
  
  def sort_dataframe_by_index(df: pd.DataFrame) -> pd.DataFrame:
      return df.sort_index(ascending=False)