[Dataset] First example

[niansong1996]

• [x] add dependency to huggingface/dataset
• [x] have a wrapper class around huggingface's dataset for our own code?
• [x] have CNN/DM ready to use

[niansong1996]

@MuroriM There are a couple of things here for you to do here:

• [ ] add specific version in the dependencies
• [ ] add some high priority datasets and show how to use them in the demos and check if they are included in the huggingface datasets: longsumm, scisumm, multi-news

[niansong1996]

@MuroriM Have things listed here been completed yet? Can you check the boxes for the ones that have been done?