Fix loading UNet only checkpoints

[CCRcmcpe]

Describe what this pull request is trying to achieve.

As in the title.
Some examples:
https://huggingface.co/Crosstyan/BPModel/blob/main/bp_mk5.safetensors
https://huggingface.co/AnnihilationOperator/ABPModel/tree/main

[CCRcmcpe]

I did write a issue with all the backgrounds described but Github give me a f**king CONNECTION_CLOSED when I'm submitting it. Feel free if any questions.

[fractal-fumbler]

fixing my issue #6768

[AUTOMATIC1111]

I'd rather not encourage people sharing incomplete checkpoints. In any case, this doesn't get in as it is no matter what - disabling the optimization for everyone. You have the state dict at the moment when you create the model, so you can check if needed layers exist and disable stuff there on that condition.

But autodetection of SD2/SD1 config works using the text layer, and if it's missing, UI won't be able to differentiate between SD1/SD2 checkpoints, so again, I'd just rather not support people's incomplete checkpoints.

[AUTOMATIC1111]

pushed a commit that should make it possible to load load the checkpoints without disabling the optimization for everyone.