def process_fn(self, batch: Batch, buffer: ReplayBuffer,
indice: np.ndarray) -> Batch:
r"""Compute the n-step return for Q-learning targets:
.. math::
G_t = \sum_{i = t}^{t + n - 1} \gamma^{i - t}(1 - d_i)r_i +
\gamma^n (1 - d_{t + n}) \max_a Q_{old}(s_{t + n}, \arg\max_a
(Q_{new}(s_{t + n}, a)))
, where :math:`\gamma` is the discount factor,
:math:`\gamma \in [0, 1]`, :math:`d_t` is the done flag of step
:math:`t`. If there is no target network, the :math:`Q_{old}` is equal
to :math:`Q_{new}`.
"""
batch = self.compute_nstep_return(batch, buffer, indice,
self._target_q, self._gamma,
self._n_step)
if isinstance(buffer, PrioritizedReplayBuffer):
batch.update_weight = buffer.update_weight
batch.indice = indice
return batch
