Fix tiny_nerf for colab compatibility

[philipkd]

Colab removed TensorFlow 1 support, so I added install commands to make it work. I also added a command to install a needed video module.

If the diff is too messy, you can simply copy and paste this code:

# %tensorflow_version 1.x
#
# TensorFlow 1 is deprecated in Colab
# https://stackoverflow.com/questions/73215696/did-colab-suspend-tensorflow-1-x

!echo y | pip uninstall tensorflow
!pip install tensorflow-gpu==1.15
!apt install --allow-change-held-packages libcudnn7=7.4.1.5-1+cuda10.0

# required for video below
!pip install imageio-ffmpeg