[Community] OpenAI Diffusion Pipeline

**Is your feature request related to a problem? Please describe.**
It would be cool to be able to use the many OpenAI Guided Diffusion models within diffusers itself.

**Describe the solution you'd like**
An OpenAIGuidedDiffusionPipeline class to load and handle OpenAI Guided Diffusion Models

**Describe alternatives you've considered**
Simply using old Disco Diffusion or my fork DiscoStream

**Additional context**
N/A