input: Write a python function to find the largest and smallest numbers in a list.
  assert find_largest_smallest([1, -2, 9, 4, -7, 0]) == (9, -7)
  assert find_largest_smallest([10, 20, 5, 6, -5, 15]) == (20, -5)
  assert find_largest_smallest([100, 200, 300, -100, -200]) == (300, -200)
output: def find_largest_smallest(nums):
      if not nums:
          return None, None
      largest = max(nums)
      smallest = min(nums)
      return largest, smallest