Normalize Motius Pipeline.from_pretrained metadata

mm_config.json

@@ -1,6 +1,6 @@
 {
   "model_type": "motionmillion",
-  "artifact_format": "hftrainer-motionmillion-v1",
+  "artifact_format": "motius-motionmillion-v1",
   "text_model_name": "google/flan-t5-xl",
   "text_encoder": {
     "stored_in_artifact": true,
@@ -34,4 +34,4 @@
       "tie_weights": false
     }
   }
-}
+}