`enable_model_cpu_offload` (#2285)
* enable_model_offload PoC
It's surprisingly more involved than expected, see comments in the PR.
* Rename final_offload_hook
* Invoke the vae forward hook manually.
* Completely remove decoder.
* Style
* apply_forward_hook decorator
* Rename method.
* Style
* Copy enable_model_cpu_offload
* Fix copies.
* Remove comment.
* Fix copies
* Missing import
* Fix doc-builder style.
* Merge main and fix again.
* Add docs
* Fix docs.
* Add a couple of tests.
* style
