Split transformers.py into multiple python files

[santiatpml]

transformers.py is getting big with multiple modules: transform, embed, fine-tuning etc. It is time to split this into multiple files so that we can iterate and test independently.

[hobson]

this transformers.py file might be naturally split into loaders, runners, and metrics. Runners could split into tasks like embedders and tex2text or vec2text generators. I'll look into the metrics functions and see what might be broken out and unittested or doctested