input: Write a function named `sum_lambda` that returns a lambda function that calculates the sum of two variables. The lambda function should take two parameters `a` and `b`, and should return their sum. Provide assert tests to verify the correctness of the lambda function.
  
  assert sum_lambda()(3, 4) == 7
  assert sum_lambda()(5, 7) == 12
  assert sum_lambda()(-1, 1) == 0
  assert sum_lambda()(0, 0) == 0
output: def sum_lambda():
      return lambda a, b: a + b