input: Write a python function to calculate the dot product of two vectors using the given Vector class.
  assert dot_product(Vector(1, 2), Vector(3, 4)) == 11
  assert dot_product(Vector(0, 0), Vector(1, 1)) == 0
  assert dot_product(Vector(-1, -2), Vector(3, 4)) == -11
output: class Vector:
      def __init__(self, x, y):
          self.x = x
          self.y = y
  
  def dot_product(v1, v2):
      return v1.x * v2.x + v1.y * v2.y