input: Write a python function to calculate the average grade from a list of grades.
  assert calculate_average_grade([90, 85, 80, 75, 70]) == 80.0
  assert calculate_average_grade([100, 95, 90, 85, 80]) == 90.0
  assert calculate_average_grade([60, 70, 80, 90, 100]) == 80.0
output: def calculate_average_grade(grades):
      return sum(grades) / len(grades)