input: Write a python function to get the number of instances created from the Counter class.
  assert Counter.get_count() == 0
  c1 = Counter()
  assert Counter.get_count() == 1
  c2 = Counter()
  c3 = Counter()
  assert Counter.get_count() == 3
output: class Counter:
      _count = 0
      
      def __init__(self):
          Counter._count += 1
  
      @classmethod
      def get_count(cls):
          return cls._count