input: Write a python function to calculate the average of two numbers using lambda function.
  assert calculate_average(4, 8) == 6
  assert calculate_average(10, 20) == 15
  assert calculate_average(5, 5) == 5
output: def calculate_average(a, b):
      average = lambda x, y: (x + y) / 2
      return average(a, b)