Fix convert_diffusers20_original_sd.py and add --variant option for loading

[Disty0]

Fix tools/convert_diffusers20_original_sd.py so it works with the latest model_util.save_stable_diffusion_checkpoint function.
Add --metadata option for model_util.save_stable_diffusion_checkpoint compatibility.
Add --variant option to specify a variant when loading a Diffusers model.
Fix 2 ruff errors: f-string without any placeholders

[kohya-ss]

Thank you for this! Sorry I forgot updating the conversion script. This is really necessary.