docs: import TensorFlow in `migrate.ipynb` (#3462)
docs: import TensorFlow in `migrate.ipynb` (#3462)

This notebook was broken because it didn't have an `import tensorflow as tf`.

TF2 is the default in Colab now, so the `%tensorflow_version 2.x`
directive is removed as well.

tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard,tensorflow/tensorboard