SDXL Support

[ljleb]

These parts of the code need to be upgraded for this to work:

• sd_mecha/weight.py relies exclusively on SD15 keys currently. We can add another mapping for SDXL and then new block weight/class weight methods for the 2 different text encoders and unet.
• the key triage in sd_mecha/merge_scheduler.py using is_passthrough_key and is_merge_key solely relies on SD1.5 key matching. This needs to change depending on whether the entire merge is SD1.5 or SDXL based.

I think that covers every location.