input: Write a function to compute the gradient, given a set of weights.
  assert compute_gradient([1, 2, 3]) == [2, 4, 6]
  assert compute_gradient([4, 5, 6]) == [8, 10, 12]
  assert compute_gradient([0, 0, 0]) == [0, 0, 0]
output: def compute_gradient(weights):
      return [2 * w for w in weights]