Encodes observations as PNG.

def _encode_observations(self, observations):
    """Encodes observations as PNG."""
    return [
        Observation(
            self._session.obj.run(
                self._encoded_image_t.obj,
                feed_dict={self._decoded_image_p.obj: observation}
            ),
            self._decode_png
        )
        for observation in observations
    ]