input: Write a function to calculate the sum of two variables using lambda. Return the lambda function. If the input variables are a and b, return a lambda function that takes a and b as arguments and returns their sum.
  assert calculate_sum(3, 4)() == 7
  assert calculate_sum(0, 0)() == 0
  assert calculate_sum(-2, 5)() == 3
  assert calculate_sum(10, -10)() == 0
  assert calculate_sum(1, 1)() == 2
output: def calculate_sum(a, b):
      return lambda: a + b