[WIP] Add FlashVideo Text-to-Video Pipeline

[dg845]

What does this PR do?

This PR adds a pipeline for the FlashVideo (paper, code, weights) text-to-video model. FlashVideo is a two-stage text-to-video model based on CogVideoX that consists of a low-resolution video generation stage followed by a low-resolution to high-resolution upscaling model.

The FlashVideo model was requested in #10767.

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@a-r-r-o-w
@yiyixuxu
@ghunkins

[a-r-r-o-w]

Thanks for working on this @dg845! Great to see you active again

Happy to review once you think it's ready & help with anything 🤗