input: Write a function to create a matrix of given dimensions n x m initialized with zeros.
  assert create_matrix(2, 3) == [[0, 0, 0], [0, 0, 0]]
  assert create_matrix(3, 3) == [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
  assert create_matrix(1, 5) == [[0, 0, 0, 0, 0]]
output: def create_matrix(n, m):
      return [[0 for _ in range(m)] for _ in range(n)]