Update config.json of UNet to include `attention_head_dim`

unet/config.json

@@ -2,6 +2,7 @@
   "_class_name": "I2VGenXLUNet",
   "_diffusers_version": "0.26.1",
   "_name_or_path": "i2vgen-xl/unet",
+  "attention_head_dim": 64,
   "block_out_channels": [
     320,
     640,