[Advanced Diffusion Script] Add Widget default text (#6100)

add widget

Author: apolinario

Diff for: examples/advanced_diffusion_training/train_dreambooth_lora_sdxl_advanced.py

@@ -157,6 +157,8 @@ def save_model_card(
 base_model: {base_model}
 instance_prompt: {instance_prompt}
 license: openrail++
+widget:
+    - text: '{validation_prompt if validation_prompt else instance_prompt}'
 ---
 """