add the class for datasets; cnndm as an example for hf dataset, and s…

[niansong1996]

Made the following change for datasets:

• added the base class for datasets in st_dataset.py;
• Added CNN/DM as an example for creating a wrapper for datasets that exist within huggingface;
• [WIP] Add ScisummNet as an example for download and process non-huggingface dataset;
• added corresponding tests for datasets module, CNN/DM is passing the test now
• Changed the model_tests.py a little to match the new dataset APIs

@MuroriM Can you follow the TODOs I made and complete the rest of the work for ScisummNet? Then we can start adding more datasets.

[niansong1996]

@MuroriM Where are we in this thread? Do we have the ScisummNet done yet? We are quite behind schedule on the dataset front, do you need more help with this?