Return a randomized batch of experiences

        # Argument
            batch_size (int): Size of the all batch
            batch_idxs (int): Indexes to extract
        # Returns
            A list of experiences randomly selected

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

        # Argument
            batch_size (int): Size of the all batch
            batch_idxs (int): Indexes to extract
        # Returns
            A list of experiences randomly selected
        """
        # It is not possible to tell whether the first state in the memory is terminal, because it
        # would require access to the "terminal" flag associated to the previous state. As a result
        # we will never return this first state (only using `self.terminals[0]` to know whether the
        # second state is terminal).
        # In addition we need enough entries to fill the desired window length.
        assert self.nb_entries >= self.window_length + 2, 'not enough entries in the memory'

        if batch_idxs is None:
            # Draw random indexes such that we have enough entries before each index to fill the
            # desired window length.
            batch_idxs = sample_batch_indexes(
                self.window_length, self.nb_entries - 1, size=batch_size)
        batch_idxs = np.array(batch_idxs) + 1
        assert np.min(batch_idxs) >= self.window_length + 1
        assert np.max(batch_idxs) < self.nb_entries
        assert len(batch_idxs) == batch_size

        # Create experiences
        experiences = []
        for idx in batch_idxs:
            terminal0 = self.terminals[idx - 2]
            while terminal0:
                # Skip this transition because the environment was reset here. Select a new, random
                # transition and use this instead. This may cause the batch to contain the same
                # transition twice.
                idx = sample_batch_indexes(self.window_length + 1, self.nb_entries, size=1)[0]
                terminal0 = self.terminals[idx - 2]
            assert self.window_length + 1 <= idx < self.nb_entries

            # This code is slightly complicated by the fact that subsequent observations might be
            # from different episodes. We ensure that an experience never spans multiple episodes.
            # This is probably not that important in practice but it seems cleaner.
            state0 = [self.observations[idx - 1]]
            for offset in range(0, self.window_length - 1):
                current_idx = idx - 2 - offset
                assert current_idx >= 1
                current_terminal = self.terminals[current_idx - 1]
                if current_terminal and not self.ignore_episode_boundaries:
                    # The previously handled observation was terminal, don't add the current one.
                    # Otherwise we would leak into a different episode.
                    break
                state0.insert(0, self.observations[current_idx])
            while len(state0) < self.window_length:
                state0.insert(0, zeroed_observation(state0[0]))
            action = self.actions[idx - 1]
            reward = self.rewards[idx - 1]
            terminal1 = self.terminals[idx - 1]

            # Okay, now we need to create the follow-up state. This is state0 shifted on timestep
            # to the right. Again, we need to be careful to not include an observation from the next
            # episode if the last state is terminal.
            state1 = [np.copy(x) for x in state0[1:]]
            state1.append(self.observations[idx])

            assert len(state0) == self.window_length
            assert len(state1) == len(state0)
            experiences.append(Experience(state0=state0, action=action, reward=reward,
                                          state1=state1, terminal1=terminal1))
        assert len(experiences) == batch_size
        return experiences