TensorFlow Object Detection API

Note to our users: the Tensorflow Object Detection API is no longer being maintained to be compatible with new versions of external dependencies (from pip, apt-get etc.). Any changes that follow are meant for internal maintenance. We may use the OD API to release projects in the future, in which case we will provide full install instructions or Docker images. We encourage users seeking an actively maintained detection / segmentation codebase to consider TF-Vision or scenic.