Openai (#905)

* add cost budget; move loc of make_dir

* support openai completion

* install pytest in workflow

* skip openai test

* test openai

* path for docs rebuild

* install datasets

* signal

* notebook

* notebook in workflow

* optional arguments and special params

* key -> k

* improve readability

* assumption

* optimize for model selection

* larger range of max_tokens

* notebook

* python package workflow

* skip on win