eval_mode in RainbowAgent

version : `dopamine-rl  3.0.1`
is there a reason why `eval_mode` is not provided as an argument in [rainbow_agent](https://github.com/google/dopamine/blob/master/dopamine/agents/rainbow/rainbow_agent.py).I see that its used [here](https://github.com/google/dopamine/blob/e7d780d7c80954b7c396d984325002d60557f7d1/dopamine/agents/rainbow/rainbow_agent.py#L325) though.