Write a function that takes an integer n as a parameter, and returns an n × n grid (list of lists) in which all cells on and below the main diagonal contain a 1 and the rest of the cells contain a 0. For example, if n = 5, your function should return the grid

[[1, 0, 0, 0, 0],
[1, 1, 0, 0, 0],
[1, 1, 1, 0, 0],
[1, 1, 1, 1, 0],
[1, 1, 1, 1, 1]]