input: Write a python function to sum all elements in a 2D list.
  assert sum_elements([[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == 45
  assert sum_elements([[0, 0, 0], [0, 0, 0], [0, 0, 0]]) == 0
  assert sum_elements([[1, 2], [3, 4], [5, 6]]) == 21
output: def sum_elements(matrix):
      return sum(sum(row) for row in matrix)