input: Write a function to sort a list of tuples by the second element (grade) in ascending order.
  assert sort_by_grade([("Person 1", 75), ("Person 2", 82), ("Person 3", 95), ("Person 4", 76)])==[('Person 1', 75), ('Person 4', 76), ('Person 2', 82), ('Person 3', 95)]
  assert sort_by_grade([("Person A", 50), ("Person B", 60), ("Person C", 40), ("Person D", 70)])==[('Person C', 40), ('Person A', 50), ('Person B', 60), ('Person D', 70)]
  assert sort_by_grade([("Alice", 88), ("Bob", 99), ("Charlie", 77)])==[('Charlie', 77), ('Alice', 88), ('Bob', 99)]
output: def sort_by_grade(lst):
      return sorted(lst, key=lambda x: x[1])