Add alternate (configurable) launcher / orchestration + sweep functionality

[haileyschoelkopf]

People often need to build their own launching scripts to run many evals / get full access to certain args / loop through models or tasks.

We should investigate adding an alternate entrypoint to the current main.py which can provide more control and remove the need for this.

Things which should be supported include:

• all CLI functionality
• control over all args to simple_evaluate() or evaluate()
• The use of external tasks or LM classes
• running over multiple settings: e.g. many different fewshot values, multiple different models, …

perhaps useful to model off of : Torchtune allows for import of arbitrary classes and orchestration of different “recipes”. https://github.com/pytorch/torchtune/blob/main/recipes/configs/alpaca_generate.yaml though we’d want a single recipe that can be used generally