Add a specified tensorflow_version to mask_rcnn_demo.ipynb.
Add a specified tensorflow_version to mask_rcnn_demo.ipynb.

Why? Colab will soon update the default version of tensorflow to 2.1.0. In order for this notebook to continue to work, I'm adding a line magic that will ensure this notebook continues to use tensorflow 1.x and execute without errors.

PiperOrigin-RevId: 286647125

tensorflow/tpu,tensorflow/tpu,tensorflow/tpu,tensorflow/tpu