input: Write a python function to check if all elements in a list are unique.
  assert is_unique([1, 2, 3, 4, 5]) == True
  assert is_unique([1, 2, 2, 3, 4]) == False
  assert is_unique([10, 20, 30, 40, 50]) == True
  assert is_unique([5, 5, 5, 5, 5]) == False
output: def is_unique(lst):
      return len(lst) == len(set(lst))