Remove logger warnings for attention backends and hard error during runtime instead

src/diffusers/models/attention_dispatch.py

@@ -38,26 +38,37 @@
 from ..utils.constants import DIFFUSERS_ATTN_BACKEND, DIFFUSERS_ATTN_CHECKS
 
 
-logger = get_logger(__name__)  # pylint: disable=invalid-name
-
-
-if is_flash_attn_available() and is_flash_attn_version(">=", "2.6.3"):
+_REQUIRED_FLASH_VERSION = "2.6.3"
+_REQUIRED_SAGE_VERSION = "2.1.1"
+_REQUIRED_FLEX_VERSION = "2.5.0"
+_REQUIRED_XLA_VERSION = "2.2"
+_REQUIRED_XFORMERS_VERSION = "0.0.29"
+
+_CAN_USE_FLASH_ATTN = is_flash_attn_available() and is_flash_attn_version(">=", _REQUIRED_FLASH_VERSION)
+_CAN_USE_FLASH_ATTN_3 = is_flash_attn_3_available()
+_CAN_USE_SAGE_ATTN = is_sageattention_available() and is_sageattention_version(">=", _REQUIRED_SAGE_VERSION)
+_CAN_USE_FLEX_ATTN = is_torch_version(">=", _REQUIRED_FLEX_VERSION)
+_CAN_USE_NPU_ATTN = is_torch_npu_available()
+_CAN_USE_XLA_ATTN = is_torch_xla_available() and is_torch_xla_version(">=", _REQUIRED_XLA_VERSION)
+_CAN_USE_XFORMERS_ATTN = is_xformers_available() and is_xformers_version(">=", _REQUIRED_XFORMERS_VERSION)
+
+
+if _CAN_USE_FLASH_ATTN:
     from flash_attn import flash_attn_func, flash_attn_varlen_func
 else:
-    logger.warning("`flash-attn` is not available or the version is too old. Please install `flash-attn>=2.6.3`.")
     flash_attn_func = None
     flash_attn_varlen_func = None
 
 
-if is_flash_attn_3_available():
+if _CAN_USE_FLASH_ATTN_3:
     from flash_attn_interface import flash_attn_func as flash_attn_3_func
     from flash_attn_interface import flash_attn_varlen_func as flash_attn_3_varlen_func
 else:
     flash_attn_3_func = None
     flash_attn_3_varlen_func = None
 
 
-if is_sageattention_available() and is_sageattention_version(">=", "2.1.1"):
+if _CAN_USE_SAGE_ATTN:
     from sageattention import (
         sageattn,
         sageattn_qk_int8_pv_fp8_cuda,
@@ -67,9 +78,6 @@
         sageattn_varlen,
     )
 else:
-    logger.warning(
-        "`sageattention` is not available or the version is too old. Please install `sageattention>=2.1.1`."
-    )
     sageattn = None
     sageattn_qk_int8_pv_fp16_cuda = None
     sageattn_qk_int8_pv_fp16_triton = None
@@ -78,39 +86,39 @@
     sageattn_varlen = None
 
 
-if is_torch_version(">=", "2.5.0"):
+if _CAN_USE_FLEX_ATTN:
     # We cannot import the flex_attention function from the package directly because it is expected (from the
     # pytorch documentation) that the user may compile it. If we import directly, we will not have access to the
     # compiled function.
     import torch.nn.attention.flex_attention as flex_attention
 
 
-if is_torch_npu_available():
+if _CAN_USE_NPU_ATTN:
     from torch_npu import npu_fusion_attention
 else:
     npu_fusion_attention = None
 
 
-if is_torch_xla_available() and is_torch_xla_version(">", "2.2"):
+if _CAN_USE_XLA_ATTN:
     from torch_xla.experimental.custom_kernel import flash_attention as xla_flash_attention
 else:
     xla_flash_attention = None
 
 
-if is_xformers_available() and is_xformers_version(">=", "0.0.29"):
+if _CAN_USE_XFORMERS_ATTN:
     import xformers.ops as xops
 else:
-    logger.warning("`xformers` is not available or the version is too old. Please install `xformers>=0.0.29`.")
     xops = None
 
 
+logger = get_logger(__name__)  # pylint: disable=invalid-name
+
 # TODO(aryan): Add support for the following:
 # - Sage Attention++
 # - block sparse, radial and other attention methods
 # - CP with sage attention, flex, xformers, other missing backends
 # - Add support for normal and CP training with backends that don't support it yet
 
-
 _SAGE_ATTENTION_PV_ACCUM_DTYPE = Literal["fp32", "fp32+fp32"]
 _SAGE_ATTENTION_QK_QUANT_GRAN = Literal["per_thread", "per_warp"]
 _SAGE_ATTENTION_QUANTIZATION_BACKEND = Literal["cuda", "triton"]
@@ -179,13 +187,16 @@ def list_backends(cls):
 
 
 @contextlib.contextmanager
-def attention_backend(backend: AttentionBackendName = AttentionBackendName.NATIVE):
+def attention_backend(backend: Union[str, AttentionBackendName] = AttentionBackendName.NATIVE):
     """
     Context manager to set the active attention backend.
     """
     if backend not in _AttentionBackendRegistry._backends:
         raise ValueError(f"Backend {backend} is not registered.")
 
+    backend = AttentionBackendName(backend)
+    _check_attention_backend_requirements(backend)
+
     old_backend = _AttentionBackendRegistry._active_backend
     _AttentionBackendRegistry._active_backend = backend
 
@@ -226,9 +237,10 @@ def dispatch_attention_fn(
         "dropout_p": dropout_p,
         "is_causal": is_causal,
         "scale": scale,
-        "enable_gqa": enable_gqa,
         **attention_kwargs,
     }
+    if is_torch_version(">=", "2.5.0"):
+        kwargs["enable_gqa"] = enable_gqa
 
     if _AttentionBackendRegistry._checks_enabled:
         removed_kwargs = set(kwargs) - set(_AttentionBackendRegistry._supported_arg_names[backend_name])
@@ -305,6 +317,57 @@ def _check_shape(
 # ===== Helper functions =====
 
 
+def _check_attention_backend_requirements(backend: AttentionBackendName) -> None:
+    if backend in [AttentionBackendName.FLASH, AttentionBackendName.FLASH_VARLEN]:
+        if not _CAN_USE_FLASH_ATTN:
+            raise RuntimeError(
+                f"Flash Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `flash-attn>={_REQUIRED_FLASH_VERSION}`."
+            )
+
+    elif backend in [AttentionBackendName._FLASH_3, AttentionBackendName._FLASH_VARLEN_3]:
+        if not _CAN_USE_FLASH_ATTN_3:
+            raise RuntimeError(
+                f"Flash Attention 3 backend '{backend.value}' is not usable because of missing package or the version is too old. Please build FA3 beta release from source."
+            )
+
+    elif backend in [
+        AttentionBackendName.SAGE,
+        AttentionBackendName.SAGE_VARLEN,
+        AttentionBackendName._SAGE_QK_INT8_PV_FP8_CUDA,
+        AttentionBackendName._SAGE_QK_INT8_PV_FP8_CUDA_SM90,
+        AttentionBackendName._SAGE_QK_INT8_PV_FP16_CUDA,
+        AttentionBackendName._SAGE_QK_INT8_PV_FP16_TRITON,
+    ]:
+        if not _CAN_USE_SAGE_ATTN:
+            raise RuntimeError(
+                f"Sage Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `sageattention>={_REQUIRED_SAGE_VERSION}`."
+            )
+
+    elif backend == AttentionBackendName.FLEX:
+        if not _CAN_USE_FLEX_ATTN:
+            raise RuntimeError(
+                f"Flex Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `torch>=2.5.0`."
+            )
+
+    elif backend == AttentionBackendName._NATIVE_NPU:
+        if not _CAN_USE_NPU_ATTN:
+            raise RuntimeError(
+                f"NPU Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `torch_npu`."
+            )
+
+    elif backend == AttentionBackendName._NATIVE_XLA:
+        if not _CAN_USE_XLA_ATTN:
+            raise RuntimeError(
+                f"XLA Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `torch_xla>={_REQUIRED_XLA_VERSION}`."
+            )
+
+    elif backend == AttentionBackendName.XFORMERS:
+        if not _CAN_USE_XFORMERS_ATTN:
+            raise RuntimeError(
+                f"Xformers Attention backend '{backend.value}' is not usable because of missing package or the version is too old. Please install `xformers>={_REQUIRED_XFORMERS_VERSION}`."
+            )
+
+
 @functools.lru_cache(maxsize=128)
 def _prepare_for_flash_attn_or_sage_varlen_without_mask(
     batch_size: int,

src/diffusers/models/modeling_utils.py

@@ -622,19 +622,21 @@ def set_attention_backend(self, backend: str) -> None:
                 attention as backend.
         """
         from .attention import AttentionModuleMixin
-        from .attention_dispatch import AttentionBackendName
+        from .attention_dispatch import AttentionBackendName, _check_attention_backend_requirements
 
         # TODO: the following will not be required when everything is refactored to AttentionModuleMixin
         from .attention_processor import Attention, MochiAttention
 
+        logger.warning("Attention backends are an experimental feature and the API may be subject to change.")
+
         backend = backend.lower()
         available_backends = {x.value for x in AttentionBackendName.__members__.values()}
         if backend not in available_backends:
             raise ValueError(f"`{backend=}` must be one of the following: " + ", ".join(available_backends))
-
         backend = AttentionBackendName(backend)
-        attention_classes = (Attention, MochiAttention, AttentionModuleMixin)
+        _check_attention_backend_requirements(backend)
 
+        attention_classes = (Attention, MochiAttention, AttentionModuleMixin)
         for module in self.modules():
             if not isinstance(module, attention_classes):
                 continue
@@ -651,6 +653,8 @@ def reset_attention_backend(self) -> None:
         from .attention import AttentionModuleMixin
         from .attention_processor import Attention, MochiAttention
 
+        logger.warning("Attention backends are an experimental feature and the API may be subject to change.")
+
         attention_classes = (Attention, MochiAttention, AttentionModuleMixin)
         for module in self.modules():
             if not isinstance(module, attention_classes):