input: Write a function to sort a list of students by their grades in descending order.
  assert sort_students([('John', 85.5), ('Chris', 90.5), ('Jane', 95.7)]) == [('Jane', 95.7), ('Chris', 90.5), ('John', 85.5)]
  assert sort_students([('Anna', 78.2), ('Paul', 88.4), ('George', 92.3)]) == [('George', 92.3), ('Paul', 88.4), ('Anna', 78.2)]
  assert sort_students([('Amy', 88.8), ('Tom', 88.8), ('Eva', 66.6)]) == [('Amy', 88.8), ('Tom', 88.8), ('Eva', 66.6)]
output: def sort_students(students):
      return sorted(students, key=lambda x: x[1], reverse=True)