Updates the policy.

def update_policy(self):
        """
        Updates the policy.
        """
        self.demonstration_buffer.update_buffer.shuffle()
        batch_losses = []
        num_batches = min(len(self.demonstration_buffer.update_buffer['actions']) //
                          self.n_sequences, self.batches_per_epoch)
        for i in range(num_batches):
            update_buffer = self.demonstration_buffer.update_buffer
            start = i * self.n_sequences
            end = (i + 1) * self.n_sequences
            mini_batch = update_buffer.make_mini_batch(start, end)
            run_out = self.policy.update(mini_batch, self.n_sequences)
            loss = run_out['policy_loss']
            batch_losses.append(loss)
        if len(batch_losses) > 0:
            self.stats['Losses/Cloning Loss'].append(np.mean(batch_losses))
        else:
            self.stats['Losses/Cloning Loss'].append(0)