input: Write a function to sort a list of numbers by their frequency in descending order.
  assert sort_by_freq([4, 5, 6, 5, 4, 3, 2, 1, 4, 6, 6, 6]) == [6, 4, 5, 3, 2, 1]
  assert sort_by_freq([1, 2, 3, 4, 5]) == [1, 2, 3, 4, 5]
  assert sort_by_freq([10, 10, 20, 30, 20, 30, 30, 30]) == [30, 10, 20]
output: def sort_by_freq(lst):
      from collections import Counter
      freq = Counter(lst)
      return [item for item, count in freq.most_common()]