COCO 🥥 on the 🤗 Hub?
This is a draft PR since I ran into few small problems. 
I referred to this TFDS code: https://github.com/tensorflow/datasets/blob/2538a08c184d53b37bfcf52cc21dd382572a88f4/tensorflow_datasets/object_detection/coco.py
cc: @mariosasko 