Wan 2.2 consists of two transformer stages.
Diffusers currently allows to run either fully on first stage or at the boundary switch to second stage (that concept somewhat resembles old-sdxl-refiner).

However, after some testing of Wan2.2, its far more useful to be able to skip first stage and run on second stage only - and right now that is not possible in Diffusers as transformers cannot be None
It should be possible to run pipeline with either transformers or transformers_2 to none with very little effort.

Also add some error handling as right now if boundary is reached, it attempts to execute second stage regardless if its loaded or not.

Also, due to extreme memory requirements, it would be highly beneficial to be able to load second stage only when its needed, not before processing even starts. Which means there should be a break-of-a-sort when first stage completes and before switching processing to second stage - perhaps via callback?

cc @yiyixuxu @a-r-r-o-w