input: Write a python function to perform bubble sort in descending order.
  assert bubble_sort([3, 5, 7, 2, 4, 1]) == [7, 5, 4, 3, 2, 1]
  assert bubble_sort([10, 2, 8, 6, 7, 3]) == [10, 8, 7, 6, 3, 2]
  assert bubble_sort([1, 3, 2, 4, 6, 5]) == [6, 5, 4, 3, 2, 1]
output: def bubble_sort(arr):
      n = len(arr)
      for i in range(n):
          for j in range(0, n-i-1):
              if arr[j] < arr[j+1]:
                  arr[j], arr[j+1] = arr[j+1], arr[j]
      return arr