diffusers

1f6ac1c3 - fix: graceful fallback when attention backends fail to import (#13060)

fix: graceful fallback when attention backends fail to import (#13060) * fix: graceful fallback when attention backends fail to import ## Problem External attention backends (flash_attn, xformers, sageattention, etc.) may be installed but fail to import at runtime due to ABI mismatches. For example, when `flash_attn` is compiled against PyTorch 2.4 but used with PyTorch 2.8, the import fails with: ``` OSError: .../flash_attn_2_cuda.cpython-311-x86_64-linux-gnu.so: undefined symbol: _ZN3c104cuda9SetDeviceEab ``` The current code uses `importlib.util.find_spec()` to check if packages exist, but this only verifies the package is installed—not that it can actually be imported. When the import fails, diffusers crashes instead of falling back to native PyTorch attention. ## Solution Wrap all external attention backend imports in try-except blocks that catch `ImportError` and `OSError`. On failure: 1. Log a warning message explaining the issue 2. Set the corresponding `_CAN_USE_*` flag to `False` 3. Set the imported functions to `None` This allows diffusers to gracefully degrade to PyTorch's native SDPA (scaled_dot_product_attention) instead of crashing. ## Affected backends - flash_attn (Flash Attention) - flash_attn_3 (Flash Attention 3) - aiter (AMD Instinct) - sageattention (SageAttention) - flex_attention (PyTorch Flex Attention) - torch_npu (Huawei NPU) - torch_xla (TPU/XLA) - xformers (Meta xFormers) ## Testing Tested with PyTorch 2.8.0 and flash_attn 2.7.4.post1 (compiled for PyTorch 2.4). Before: crashes on import. After: logs warning and uses native attention. * address review: use single logger and catch RuntimeError - Move logger to module level instead of creating per-backend loggers - Add RuntimeError to exception list alongside ImportError and OSError Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com> * Apply style fixes --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com> Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>