Logged average_q is NaN for CategoricalDQN.get_statistics()

[keisuke-nakata]

For CategoricalDQN, logged average_q values in statistics are always nan.

average_q stats is the mean value of self.q_record.
https://github.com/pfnet/pfrl/blob/d420891573/pfrl/agents/dqn.py#L695

However, CategoricalDQN overrides _compute_loss function,
https://github.com/pfnet/pfrl/blob/d420891573/pfrl/agents/categorical_dqn.py#L172-L198
which is responsible to append a Q value to self.q_record, and thus the mean is always nan.

Obtaining scalar Q values in categorical algorithms is a bit unclear since they calculate distribution of Q values instead of scalar, but I think that taking expectation value of the distribution is natural. (as greedy_action does.)