input: Write a python function to get the current count of Employee instances.
  assert Employee.get_employee_count() == 0
  e1 = Employee()
  assert Employee.get_employee_count() == 1
  e2 = Employee()
  e3 = Employee()
  assert Employee.get_employee_count() == 3
output: class Employee:
      _employee_count = 0
  
      def __init__(self):
          Employee._employee_count += 1
  
      @classmethod
      def get_employee_count(cls):
          return cls._employee_count