Implement modelspec metadata in CheckpointSave for SDXL and refiner.

comfy_extras/nodes_model_merging.py

 import comfy.sd
 import comfy.utils
+import comfy.model_base
+
 import folder_paths
 import json
 import os
@@ -100,6 +102,31 @@ class CheckpointSave:
             prompt_info = json.dumps(prompt)
 
         metadata = {"prompt": prompt_info}
+
+
+        enable_modelspec = True
+        if isinstance(model.model, comfy.model_base.SDXL):
+            metadata["modelspec.architecture"] = "stable-diffusion-xl-v1-base"
+        elif isinstance(model.model, comfy.model_base.SDXLRefiner):
+            metadata["modelspec.architecture"] = "stable-diffusion-xl-v1-refiner"
+        else:
+            enable_modelspec = False
+
+        if enable_modelspec:
+            metadata["modelspec.sai_model_spec"] = "1.0.0"
+            metadata["modelspec.implementation"] = "sgm"
+            metadata["modelspec.title"] = "{} {}".format(filename, counter)
+
+        #TODO:
+        # "stable-diffusion-v1", "stable-diffusion-v1-inpainting", "stable-diffusion-v2-512",
+        # "stable-diffusion-v2-768-v", "stable-diffusion-v2-unclip-l", "stable-diffusion-v2-unclip-h",
+        # "v2-inpainting"
+
+        if model.model.model_type == comfy.model_base.ModelType.EPS:
+            metadata["modelspec.predict_key"] = "epsilon"
+        elif model.model.model_type == comfy.model_base.ModelType.V_PREDICTION:
+            metadata["modelspec.predict_key"] = "v"
+
         if extra_pnginfo is not None:
             for x in extra_pnginfo:
                 metadata[x] = json.dumps(extra_pnginfo[x])