Return a randomized batch of params and rewards

        # Argument
            batch_size (int): Size of the all batch
            batch_idxs (int): Indexes to extract
        # Returns
            A list of params randomly selected and a list of associated rewards

def sample(self, batch_size, batch_idxs=None):
        """Return a randomized batch of params and rewards

        # Argument
            batch_size (int): Size of the all batch
            batch_idxs (int): Indexes to extract
        # Returns
            A list of params randomly selected and a list of associated rewards
        """
        if batch_idxs is None:
            batch_idxs = sample_batch_indexes(0, self.nb_entries, size=batch_size)
        assert len(batch_idxs) == batch_size

        batch_params = []
        batch_total_rewards = []
        for idx in batch_idxs:
            batch_params.append(self.params[idx])
            batch_total_rewards.append(self.total_rewards[idx])
        return batch_params, batch_total_rewards