Of note: The Tensorflow maintainers have released TF2.x compatibility with the Object Detection API. I would recommend sticking with TF1.x for now as the new API is still being developed. Otherwise, you can see the changes in the processes here and try out the new API.
You only need to install if you are going to use model_main.py to train your model.
See my walkthrough here: 385
Of note: The Tensorflow maintainers have released TF2.x compatibility with the Object Detection API. I would recommend sticking with TF1.x for now as the new API is still being developed. Otherwise, you can see the changes in the processes here and try out the new API.