input: Write a python function to calculate the average grade for a student.
  ```python
  student = Student(1)
  student.set_grade("Math", 90)
  student.set_grade("Science", 80)
  student.set_grade("Literature", 70)
  assert student.get_average_grade() == 80.0
  
  student2 = Student(2)
  student2.set_grade("Math", 85)
  student2.set_grade("Science", 95)
  student2.set_grade("Literature", 75)
  student2.set_grade("History", 80)
  assert student2.get_average_grade() == 83.75
  
  student3 = Student(3)
  student3.set_grade("Math", 100)
  student3.set_grade("Science", 100)
  assert student3.get_average_grade() == 100.0
output: class Student:
      def __init__(self, student_id):
          self.student_id = student_id
          self.grades = {}
  
      def set_grade(self, subject, grade):
          self.grades[subject] = grade
  
      def get_average_grade(self):
          return sum(self.grades.values()) / len(self.grades)