input: Write a python function to sort a list of strings by their lengths in descending order.
  assert sort_by_length_descending(['Math', 'English', 'Computer Science', 'Physics']) == ['Computer Science', 'English', 'Physics', 'Math']
  assert sort_by_length_descending(['a', 'abc', 'ab']) == ['abc', 'ab', 'a']
  assert sort_by_length_descending(['dog', 'cat', '']) == ['dog', 'cat', '']
output: def sort_by_length_descending(strings):
      return sorted(strings, key=len, reverse=True)