Fix crash when flash_attn is installed but broken

Verify attention backend functions are actually callable before marking
them available. Falls back to PyTorch SDPA instead of calling None.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

flashvsr_arch/models/wan_video_dit.py

@@ -11,26 +11,30 @@ from .utils import hash_state_dict_keys
 
 try:
     import flash_attn_interface
+    assert callable(getattr(flash_attn_interface, "flash_attn_func", None))
     FLASH_ATTN_3_AVAILABLE = True
-except ModuleNotFoundError:
+except Exception:
     FLASH_ATTN_3_AVAILABLE = False
 
 try:
     import flash_attn
+    assert callable(getattr(flash_attn, "flash_attn_func", None))
     FLASH_ATTN_2_AVAILABLE = True
-except ModuleNotFoundError:
+except Exception:
     FLASH_ATTN_2_AVAILABLE = False
 
 try:
     from sageattention import sageattn
+    assert callable(sageattn)
     SAGE_ATTN_AVAILABLE = True
-except ModuleNotFoundError:
+except Exception:
     SAGE_ATTN_AVAILABLE = False
 
 try:
     from sageattn.core import sparse_sageattn
+    assert callable(sparse_sageattn)
     SPARSE_SAGE_AVAILABLE = True
-except ModuleNotFoundError:
+except Exception:
     SPARSE_SAGE_AVAILABLE = False
     sparse_sageattn = None
 from PIL import Image