input: Write a python function to replace each element in a matrix with its column index.
  assert replace_with_column_index([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
  assert replace_with_column_index([[3, 5, 7], [2, 4, 6], [1, 3, 5]]) == [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
  assert replace_with_column_index([[9, 8, 7], [6, 5, 4], [3, 2, 1]]) == [[0, 1, 2], [0, 1, 2], [0, 1, 2]]
output: def replace_with_column_index(matrix):
      return [[j for j in range(len(matrix[0]))] for _ in range(len(matrix))]