input: Write a python function to generate a random matrix with dimensions m by n.
  assert len(random_matrix(2, 3)) == 2
  assert len(random_matrix(2, 3)[0]) == 3
  assert all(0 <= cell <= 100 for row in random_matrix(5, 5) for cell in row)
output: import random
  
  def random_matrix(m, n):
      return [[random.randint(0, 100) for _ in range(n)] for _ in range(m)]