Reorganize code structure, fix client import bug

[mrwyattii]

This PR is a first step to better organize the MII code. Major changes include:

• Creating an api.py file for user-facing functions
• Moving client.py and server.py into backend/ (this also resolves a bug where we previously did from .client import client in our __init__.py file, which caused the mii.client submodule to be replaced with the mii.client function)
• Moving models.py and tokenizers.py into modeling/
• Moving task_methods.py under grpc_related/ (Because it is only used with the persistent GRPC deployment)

@jeffra

[jeffra]

This is great, thank you @mrwyattii. @tohtana please take a look as well but otherwise looks good to me.

[tohtana]

It looks good to me. I left small comments to share my thought, but I don't think we need to address them in this PR