Add the Initial Version of Interactive Demo

[dalgu90]

Hey team. I added an initial version of the interactive ICD coding demo.
The demo app is written by Streamlit and can run only one model for now. We can run the app (a dataset and a model needed) by running a command like

$ streamlit run app.py -- --config_path configs/demo/caml_mimic3_50.yml

While implementing the app, I also updated other things in the code:

• Refactored the base_dataset class exposing encoding/decoding methods (of both input text and label).
• Changed the type of error when the key misses in the __getattr__() of configuration (KeyError -> AttributeError).
• Changed the checkpoint saver not to change the input config.

Here's a screenshot of the demo. Please nitpick any details of the code.

[dalgu90]

Thanks for the approval @abheesht17 . I just passed the pre-commit hooks and will merge this PR.