[GH->HF] Load datasets from the Hub
Currently datasets with no namespace (e.g. squad, glue) are loaded from github.

In this PR I changed this logic to use the Hugging Face Hub instead.

This is the first step in removing all the dataset scripts in this repository

related to discussions in https://github.com/huggingface/datasets/pull/4059 (I should have continued from this PR actually)