Cartpole colab: "No module named 'tensorflow.contrib'"

Hi,

The [cartpole colab](https://colab.research.google.com/github/google/dopamine/blob/master/dopamine/colab/cartpole.ipynb) raises the below import error on the public Python 3 GCE colab backend. Probably a TF1 vs TF2 issue?

```
---------------------------------------------------------------------------
ModuleNotFoundError Traceback (most recent call last)
in ()
3 import numpy as np
4 import os
----> 5 from dopamine.discrete_domains import run_experiment
6 from dopamine.colab import utils as colab_utils
7 from absl import flags

2 frames
/usr/local/lib/python3.6/dist-packages/dopamine/discrete_domains/run_experiment.py in ()
23 import time
24
---> 25 from dopamine.agents.dqn import dqn_agent
26 from dopamine.agents.implicit_quantile import implicit_quantile_agent
27 from dopamine.agents.rainbow import rainbow_agent

/usr/local/lib/python3.6/dist-packages/dopamine/agents/dqn/dqn_agent.py in ()
25
26
---> 27 from dopamine.discrete_domains import atari_lib
28 from dopamine.replay_memory import circular_replay_buffer
29 import numpy as np

/usr/local/lib/python3.6/dist-packages/dopamine/discrete_domains/atari_lib.py in ()
52
53 import cv2
---> 54 from tensorflow.contrib import layers as contrib_layers
55 from tensorflow.contrib import slim as contrib_slim
56

ModuleNotFoundError: No module named 'tensorflow.contrib'

---------------------------------------------------------------------------
NOTE: If your import is failing due to a missing package, you can
manually install dependencies using either !pip or !apt.

To view examples of installing some common dependencies, click the
"Open Examples" button below.
---------------------------------------------------------------------------```

Start with dopamine/colab/cartpole.ipynb and reproduce the import failure on the public Python 3 Colab backend. Then inspect dopamine/discrete_domains/atari_lib.py, especially its tensorflow.contrib imports, alongside the notebook's dependency setup. Done means the cartpole Colab imports successfully without the reported ModuleNotFoundError.

Written by the indexing model from the issue text.