input: Write a function to sort a list of strings by length in descending order.
  assert sort_by_length(['apple', 'cat', 'ball', 'giraffe', 'elephant', 'dog', 'frog'])==['elephant', 'giraffe', 'ball', 'apple', 'frog', 'dog', 'cat']
  assert sort_by_length(['banana', 'kiwi', 'mango', 'strawberry', 'cherry'])==['strawberry', 'banana', 'cherry', 'mango', 'kiwi']
  assert sort_by_length(['one', 'three', 'two', 'four'])==['three', 'four', 'one', 'two']
output: def sort_by_length(strings):
      return sorted(strings, key=len, reverse=True)