[core] use kernels to support _flash_3_hub attention backend

src/diffusers/models/attention_dispatch.py

@@ -17,6 +17,7 @@
 import inspect
 import math
 from enum import Enum
+from functools import lru_cache
 from typing import Any, Callable, Dict, List, Literal, Optional, Tuple, Union
 
 import torch
@@ -26,6 +27,7 @@
     is_flash_attn_3_available,
     is_flash_attn_available,
     is_flash_attn_version,
+    is_kernels_available,
     is_sageattention_available,
     is_sageattention_version,
     is_torch_npu_available,
@@ -131,7 +133,6 @@ def wrap(func):
     _custom_op = custom_op_no_op
     _register_fake = register_fake_no_op
 
-
 logger = get_logger(__name__)  # pylint: disable=invalid-name
 
 # TODO(aryan): Add support for the following:
@@ -153,6 +154,8 @@ class AttentionBackendName(str, Enum):
     FLASH_VARLEN = "flash_varlen"
     _FLASH_3 = "_flash_3"
     _FLASH_VARLEN_3 = "_flash_varlen_3"
+    _FLASH_3_HUB = "_flash_3_hub"
+    # _FLASH_VARLEN_3_HUB = "_flash_varlen_3_hub"  # not supported yet.
 
     # PyTorch native
     FLEX = "flex"
@@ -207,6 +210,22 @@ def list_backends(cls):
         return list(cls._backends.keys())
 
 
+@lru_cache(maxsize=None)
+def _load_fa3_hub():
+    from ..utils.kernels_utils import _get_fa3_from_hub
+
+    fa3_hub = _get_fa3_from_hub()  # won't re-download if already present
+    if fa3_hub is None:
+        raise RuntimeError(
+            "Failed to load FlashAttention-3 kernels from the Hub. Please ensure the wheel is available for your platform."
+        )
+    return fa3_hub
+
+
+def flash_attn_3_hub_func(*args, **kwargs):
+    return _load_fa3_hub().flash_attn_func(*args, **kwargs)
+
+
 @contextlib.contextmanager
 def attention_backend(backend: Union[str, AttentionBackendName] = AttentionBackendName.NATIVE):
     """
@@ -351,6 +370,13 @@ def _check_attention_backend_requirements(backend: AttentionBackendName) -> None
                 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."
             )
 
+    # TODO: add support Hub variant of FA3 varlen later
+    elif backend in [AttentionBackendName._FLASH_3_HUB]:
+        if not is_kernels_available():
+            raise RuntimeError(
+                f"Flash Attention 3 Hub backend '{backend.value}' is not usable because the `kernels` package isn't available. Please install it with `pip install kernels`."
+            )
+
     elif backend in [
         AttentionBackendName.SAGE,
         AttentionBackendName.SAGE_VARLEN,
@@ -514,6 +540,22 @@ def _(query: torch.Tensor, key: torch.Tensor, value: torch.Tensor) -> Tuple[torc
     return torch.empty_like(query), query.new_empty(lse_shape)
 
 
+@_custom_op("vllm_flash_attn3::flash_attn", mutates_args=(), device_types="cuda")
+def _wrapped_flash_attn_3_hub(
+    query: torch.Tensor, key: torch.Tensor, value: torch.Tensor
+) -> Tuple[torch.Tensor, torch.Tensor]:
+    out, lse = flash_attn_3_hub_func(query, key, value)
+    lse = lse.permute(0, 2, 1)
+    return out, lse
+
+
+@_register_fake("vllm_flash_attn3::flash_attn")
+def _(query: torch.Tensor, key: torch.Tensor, value: torch.Tensor) -> Tuple[torch.Tensor, torch.Tensor]:
+    batch_size, seq_len, num_heads, head_dim = query.shape
+    lse_shape = (batch_size, seq_len, num_heads)
+    return torch.empty_like(query), query.new_empty(lse_shape)
+
+
 # ===== Attention backends =====
 
 
@@ -657,6 +699,41 @@ def _flash_attention_3(
     return (out, lse) if return_attn_probs else out
 
 
+@_AttentionBackendRegistry.register(
+    AttentionBackendName._FLASH_3_HUB,
+    constraints=[_check_device, _check_qkv_dtype_bf16_or_fp16, _check_shape],
+)
+def _flash_attention_3_hub(
+    query: torch.Tensor,
+    key: torch.Tensor,
+    value: torch.Tensor,
+    scale: Optional[float] = None,
+    is_causal: bool = False,
+    window_size: Tuple[int, int] = (-1, -1),
+    softcap: float = 0.0,
+    deterministic: bool = False,
+    return_attn_probs: bool = False,
+) -> torch.Tensor:
+    out, lse, *_ = flash_attn_3_hub_func(
+        q=query,
+        k=key,
+        v=value,
+        softmax_scale=scale,
+        causal=is_causal,
+        qv=None,
+        q_descale=None,
+        k_descale=None,
+        v_descale=None,
+        window_size=window_size,
+        softcap=softcap,
+        num_splits=1,
+        pack_gqa=None,
+        deterministic=deterministic,
+        sm_margin=0,
+    )
+    return (out, lse) if return_attn_probs else out
+
+
 @_AttentionBackendRegistry.register(
     AttentionBackendName._FLASH_VARLEN_3,
     constraints=[_check_device, _check_qkv_dtype_bf16_or_fp16, _check_shape],

src/diffusers/utils/kernels_utils.py

@@ -0,0 +1,22 @@
+from ..utils import get_logger
+from .import_utils import is_kernels_available
+
+
+logger = get_logger(__name__)
+
+
+_DEFAULT_HUB_ID_FA3 = "kernels-community/vllm-flash-attn3"
+
+
+def _get_fa3_from_hub():
+    if not is_kernels_available():
+        return None
+    else:
+        from kernels import get_kernel
+
+        try:
+            flash_attn_3_hub = get_kernel(_DEFAULT_HUB_ID_FA3)
+            return flash_attn_3_hub
+        except Exception as e:
+            logger.error(f"An error occurred while fetching kernel '{_DEFAULT_HUB_ID_FA3}' from the Hub: {e}")
+            raise