Add explicit tensorflow version to deeplab_demo.ipynb (#7964)

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.

research/deeplab/deeplab_demo.ipynb

@@ -61,6 +61,7 @@
         "import numpy as np\n",
         "from PIL import Image\n",
         "\n",
+        "%tensorflow_version 1.x\n",
         "import tensorflow as tf"
       ]
     },