Update app.py: set ATTN_BACKEND to sdpa

app.py

@@ -30,6 +30,9 @@
 import gradio as gr
 import os
 os.environ['SPCONV_ALGO'] = 'native'
+# Hi3DGen uses an attention backend that defaults to 'xformers', which requires an extra
+# dependency not installed in this Space.  Override the backend to use 'sdpa' instead.
+os.environ['ATTN_BACKEND'] = 'sdpa'
 from typing import *
 import torch
 import numpy as np