input: Write a function to count the number of unique elements in an integer array. Return the count of unique elements.
  assert num_unique_elements([1, 2, 3, 2, 1, 4, 7, 8, 9, 8]) == 7
  assert num_unique_elements([5, 5, 5, 5]) == 1
  assert num_unique_elements([]) == 0
  assert num_unique_elements([10, 20, 30, 10]) == 3
  assert num_unique_elements([7, 7, 7, 7, 8, 8]) == 2
output: def num_unique_elements(arr):
      return len(set(arr))