分别定义Person类、Employee类、Student类以及在职研究生类PartTime Graduate Student，并且画出相应的继承层次图。

以下是定义的代码和继承层次图：

class Person:
    def __init__(self, name, age):
        self.name = name
        self.age = age
        
    def introduce(self):
        print(f"Hi, my name is {self.name} and I am {self.age} years old.")
        

class Employee(Person):
    def __init__(self, name, age, salary):
        super().__init__(name, age)
        self.salary = salary
        
    def work(self):
        print(f"{self.name} is working hard.")
        

class Student(Person):
    def __init__(self, name, age, school):
        super().__init__(name, age)
        self.school = school
        
    def study(self):
        print(f"{self.name} is studying at {self.school}.")
        

class PartTimeGraduateStudent(Student):
    def __init__(self, name, age, school, company):
        super().__init__(name, age, school)
        self.company = company
        
    def work_and_study(self):
        print(f"{self.name} is working at {self.company} and studying at {self.school}.")

继承层次图：

        +--------+
        | Person |
        +--------+
            |
     +------+------+
     |             |
+---------+   +---------+
| Employee|   | Student |
+---------+   +---------+
                |
        +---------------+
        | PartTimeGraduate|
        +---------------+