    def sample(self, batch_size, device):
        """
        Importance-sample timesteps for a batch.

        :param batch_size: the number of timesteps.
        :param device: the torch device to save to.
        :return: a tuple (timesteps, weights):
                 - timesteps: a tensor of timestep indices.
                 - weights: a tensor of weights to scale the resulting losses.
        """
        w = self.weights()
        p = w / np.sum(w)
        indices_np = np.random.choice(len(p), size=(batch_size,), p=p)
        indices = th.from_numpy(indices_np).long().to(device)
        weights_np = 1 / (len(p) * p[indices_np])
        weights = th.from_numpy(weights_np).float().to(device)
        return indices, weights
Importance-sample timesteps for a batch.

:param batch_size: the number of timesteps.
:param device: the torch device to save to.
:return: a tuple (timesteps, weights):
- timesteps: a tensor of timestep indices.
- weights: a tensor of weights to scale the resulting losses.
modules/diffusion/karras/sample.py