input: Write a function to calculate the sum of two variables using lambda. The function should return the lambda expression that defines the sum operation.
  assert calculate_sum()=="sum = lambda a, b : a + b\nprint(sum(3,4))"
  assert calculate_sum()=="sum = lambda a, b : a + b\nprint(sum(3,4))"
  assert calculate_sum()=="sum = lambda a, b : a + b\nprint(sum(3,4))"
  assert calculate_sum()=="sum = lambda a, b : a + b\nprint(sum(3,4))"
  assert calculate_sum()=="sum = lambda a, b : a + b\nprint(sum(3,4))"
output: return "sum = lambda a, b : a + b\nprint(sum(3,4))"