[Agents] Create agent from model file

[emilydinan]

Patch description
Make it easier to create an agent from model file. Before, we did not add a datapath or model_subargs which could make loading or overriding opts difficult. Also cleaned up the create_agent_from_opt_file function (renaming a few things and moving some stuff around). It's still a bit of a mess though TBH.

Now you can do something like:
agent = create_agent_from_model_file('zoo:blender/blender_90M/model', {'inference': 'topk'}) and it will automatically create the agent with all of the correct arguments added and override the inference to topk.

This will be useful for M-Turk and other tasks where we have to create "subagents" (like retrieve and refine). It is also incidentally useful for #2803 in which @dianaglzrico needs to create a partner_agent for two models to speak to each other.

[emilydinan]

I do not know what this mypy error is

[klshuster]

i think this all looks good, just a few questions

also mypy is failing everywhere (something about installing a dependency), so dw about it