[BOUNTY - $200] make it so process_prompt is cancellable outside of a download (follow up to #305)

[AlexCheema]

See #305

Currently the fix is to shield process_prompt however this also makes it so that if a chatgpt api request times out, it never gets cancelled even when a download is not in progress.

This can lead to behaviour where exo keeps generating even after a chatgpt api request has timed out.

Gotchas: make sure however exo works right is actually cancellable end-to-end. If an inference is in progress, that needs to be cancellable - InferenceEngines need to support this behaviour.

[ji-cryptocafe]

Started working on this. But since holidays are near, it could take a few days.