Adding 🤗nlp in the examples
This PR examines how to best make use of all the features of 🤗nlp in the examples.

First, example studied is GLUE. The main goal is to have a very explicit data processing (target: no data processing happening inside `transformers`) as well as add some efficiency features like dynamic batching. The second goal is to make this a lot more efficient, fast, and reproducible.
Closing in favor of #5240