Advent of Code 2021 in pure TensorFlow - day 9. Image gradients, 4-neighborhood, and flood fill algorithms in pure TensorFlow

The day 9 challenge can be seen as a computer vision problem. TensorFlow contains some computer vision utilities that we’ll use - like the image gradient - but it’s not a complete framework for computer vision (like OpenCV). Anyway, the framework offers primitive data types like tf.TensorArray and tf.queue that we can use for implementing a flood-fill algorithm in pure TensorFlow and solve the problem.