update openai wrapper to work with tiktoken interface and newest openai version

[bmosaicml]

Some changes to the OpenAI tokenizer (tiktoken) made the previously working e2e OpenAI eval wrapper no longer work with the eval script.

Some changes to the newest version of OpenAI API include deprecating the openai.ChatCompletion.create method in favor of first constructing a client and calling client.chat.completion.create. Additionally, return results are no longer dictionaries , but instead openai.types.Completion objects

Tested with composer eval/eval.py eval/yamls/openai_eval.yaml

Ran on all of lm_tasks_v0.2: openai-eval-O5OK4O

[dakinggg]

Thank you! Could you please add a note to the yaml noting which tasks are supported with which models? (some are not supported because we can't get logits, right?)

stale

[maxisawesome]

with the changes, LGTM 👍

[dakinggg]

Approving to unblock the setup.py change. Deferring the full review to Max.