[Educational] Typing .py files iteratively with mypy

[msaroufim]

We just added #1858 support so a good first issue for anyone new to torchserve or open source python projects in general I'd recommend the following

1. Pick any file in serve/ts
2. Add type annotations - for an example you can checkout https://github.com/pytorch/serve/blob/master/ts/context.py
3. Run mypy path_to_file_you_annotated.py
4. If you don't see any issues then make a PR and make sure the mypy lint action passes

[josephcalise]

I would love to take care of this issue on any files that need type annotations in serve/ts, are there any specific ones that are still open? I am new to contributing to open source, I have identified serve/ts/model_server.py as something I am able to complete right now, and will continue to look into the rest of the files to try to understand the types and complete any other files I can.

Thank you!