Infer policy from stack of observations.

    Args:
      ob_stack: array of shape (1, frame_stack_size, height, width, channels)

    Returns:
      logits and vf.

def infer_from_frame_stack(self, ob_stack):
    """Infer policy from stack of observations.

    Args:
      ob_stack: array of shape (1, frame_stack_size, height, width, channels)

    Returns:
      logits and vf.
    """
    logits, vf = self.sess.run([self.logits_t, self.value_function_t],
                               feed_dict={self.obs_t: ob_stack})
    return logits, vf