Operations

In this page we list our PyTorch Autograd-compatible operations. These operations come with performance knobs (configurations), some of which are specific to certain backends.

Changing those knobs is completely optional, and NATTEN will continue to be functionally correct in all cases. However, to squeeze out the maximum performance achievable, we highly recommend looking at backends, or just using our profiler toolkit and its dry run feature to navigate through available backends and their valid configurations for your specific use case and GPU architecture. You can also use the profiler's optimize feature to search and find the best configuration.

Neighborhood Attention

natten.na1d

na1d(
    query,
    key,
    value,
    kernel_size,
    stride=1,
    dilation=1,
    is_causal=False,
    scale=None,
    additional_keys=None,
    additional_values=None,
    attention_kwargs=None,
    backend=None,
    q_tile_shape=None,
    kv_tile_shape=None,
    backward_q_tile_shape=None,
    backward_kv_tile_shape=None,
    backward_kv_splits=None,
    backward_use_pt_reduction=False,
    run_persistent_kernel=True,
    kernel_schedule=None,
    torch_compile=False,
)

Computes 1-D neighborhood attention.

Parameters:

Name	Type	Description	Default
query	Tensor	4-D query tensor, with the heads last layout ([batch, seqlen, heads, head_dim])	required
key	Tensor	4-D key tensor, with the heads last layout ([batch, seqlen, heads, head_dim])	required
value	Tensor	4-D value tensor, with the heads last layout ([batch, seqlen, heads, head_dim])	required
kernel_size	Tuple[int] | int	Neighborhood window (kernel) size. Note kernel_size must be smaller than or equal to seqlen.	required
stride	Tuple[int] | int	Sliding window step size. Defaults to 1 (standard sliding window). Note stride must be smaller than or equal to kernel_size. When stride == kernel_size, there will be no overlap between sliding windows, which is equivalent to blocked attention (a.k.a. window self attention).	1
dilation	Tuple[int] | int	Dilation step size. Defaults to 1 (standard sliding window). Note The product of dilation and kernel_size must be smaller than or equal to seqlen.	1
is_causal	Tuple[bool] | bool	Toggle causal masking. Defaults to False (bi-directional).	False
scale	float	Attention scale. Defaults to head_dim ** -0.5.	None
additional_keys	Optional[Tensor]	None or 4-D key tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to key tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens.	None
additional_values	Optional[Tensor]	None or 4-D value tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to value tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens. Note additional_keys and additional_values must both either be Tensors, or both Nones, and must match in shape.	None

Other Parameters:

Name	Type	Description
backend	str	Backend implementation to run with. Choices are: None (pick the best available one), "cutlass-fna", "hopper-fna", "blackwell-fna", "flex-fna". Refer to backends for more information.
q_tile_shape	Tuple[int]	1-D Tile shape for the query token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
kv_tile_shape	Tuple[int]	1-D Tile shape for the key-value token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
backward_q_tile_shape	Tuple[int]	1-D Tile shape for the query token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_tile_shape	Tuple[int]	1-D Tile shape for the key/value token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_splits	Tuple[int]	Number of key/value tiles allowed to work in parallel in the backward pass kernel. Like tile shapes, this is a tuple and not an integer for neighborhood attention operations, and the size of the tuple corresponds to the number of dimensions / rank of the layout of tokens. This is only respected by the "cutlass-fna" backend, and only when KV parallelism is enabled.
backward_use_pt_reduction	bool	Whether to use PyTorch eager for computing the dO * O product required by the backward pass, over the CUTLASS kernel. This only applies to the "cutlass-fna" backend.
run_persistent_kernel	bool	Whether to use persistent tile scheduling in the forward pass kernel. This only applies to the "blackwell-fna" backend.
kernel_schedule	Optional[str]	Kernel type (Hopper architecture only). Choices are None: pick the default, "non" (non-persistent), "coop" (warp-specialized cooperative), or "pp" (warp-specialized ping-ponging). Refer to Hopper FMHA/FNA backend for more information.
torch_compile	bool	Applies only to the "flex-fna" backend. Whether or not to JIT compile the attention kernel. Due to this being an experimental feature in PyTorch, we do not recommend it, and it is guarded by context flags. Read more in Flex Attention + torch.compile.
attention_kwargs	Optional[Dict]	arguments to the attention operator, if used to implement neighborhood cross-attention, or self attention as a fast path for neighborhood attention. If additional_{keys,values} are specified, NATTEN usually performs a separate cross-attention using our attention operator, and merges the results. If for a given use case, the neighborhood attention problem is equivalent to self attention (not causal, kernel_size == seqlen), NATTEN will also attempt to directly use attention. You can override arguments to attention by passing a dictionary here. Example out = na1d( q, k, v, kernel_size=kernel_size, ..., attention_kwargs={ "backend": "blackwell-fmha", "run_persistent_kernel": True, } )

Returns:

Name	Type	Description
output	Tensor	4-D output tensor, with the heads last layout ([batch, seqlen, heads, head_dim]).

natten.na2d

na2d(
    query,
    key,
    value,
    kernel_size,
    stride=1,
    dilation=1,
    is_causal=False,
    scale=None,
    additional_keys=None,
    additional_values=None,
    attention_kwargs=None,
    backend=None,
    q_tile_shape=None,
    kv_tile_shape=None,
    backward_q_tile_shape=None,
    backward_kv_tile_shape=None,
    backward_kv_splits=None,
    backward_use_pt_reduction=False,
    run_persistent_kernel=True,
    kernel_schedule=None,
    torch_compile=False,
)

Computes 2-D neighborhood attention.

Parameters:

Name	Type	Description	Default
query	Tensor	2-D query tensor, with the heads last layout: [batch, X, Y, heads, head_dim], where token layout shape (feature map shape) is (X, Y).	required
key	Tensor	2-D key tensor, with the heads last layout: [batch, X, Y, heads, head_dim], where token layout shape (feature map shape) is (X, Y).	required
value	Tensor	2-D value tensor, with the heads last layout: [batch, X, Y, heads, head_dim], where token layout shape (feature map shape) is (X, Y).	required
kernel_size	Tuple[int, int] | int	Neighborhood window (kernel) size/shape. If an integer, it will be repeated for all 2 dimensions. For example kernel_size=3 is reinterpreted as kernel_size=(3, 3). Note kernel_size must be smaller than or equal to token layout shape ((X, Y)) along every dimension.	required
stride	Tuple[int, int] | int	Sliding window step size/shape. Defaults to 1 (standard sliding window). If an integer, it will be repeated for all 2 dimensions. For example stride=2 is reinterpreted as stride=(2, 2). Note stride must be smaller than or equal to kernel_size along every dimension. When stride == kernel_size, there will be no overlap between sliding windows, which is equivalent to blocked attention (a.k.a. window self attention).	1
dilation	Tuple[int, int] | int	Dilation step size/shape. Defaults to 1 (standard sliding window). If an integer, it will be repeated for all 2 dimensions. For example dilation=4 is reinterpreted as dilation=(4, 4). Note The product of dilation and kernel_size must be smaller than or equal to token layout shape ((X, Y)) along every dimension.	1
is_causal	Tuple[bool, bool] | bool	Toggle causal masking. Defaults to False (bi-directional). If a boolean, it will be repeated for all 2 dimensions. For example is_causal=True is reinterpreted as is_causal=(True, True).	False
scale	float	Attention scale. Defaults to head_dim ** -0.5.	None
additional_keys	Optional[Tensor]	None or 4-D key tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to key tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens.	None
additional_values	Optional[Tensor]	None or 4-D value tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to value tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens. Note additional_keys and additional_values must both either be Tensors, or both Nones, and must match in shape.	None

Other Parameters:

Name	Type	Description
backend	str	Backend implementation to run with. Choices are: None (pick the best available one), "cutlass-fna", "hopper-fna", "blackwell-fna", "flex-fna". Refer to backends for more information.
q_tile_shape	Tuple[int, int]	2-D Tile shape for the query token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
kv_tile_shape	Tuple[int, int]	2-D Tile shape for the key-value token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
backward_q_tile_shape	Tuple[int, int]	2-D Tile shape for the query token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_tile_shape	Tuple[int, int]	2-D Tile shape for the key/value token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_splits	Tuple[int, int]	Number of key/value tiles allowed to work in parallel in the backward pass kernel. Like tile shapes, this is a tuple and not an integer for neighborhood attention operations, and the size of the tuple corresponds to the number of dimensions / rank of the layout of tokens. This is only respected by the "cutlass-fna" backend, and only when KV parallelism is enabled.
backward_use_pt_reduction	bool	Whether to use PyTorch eager for computing the dO * O product required by the backward pass, over the CUTLASS kernel. This only applies to the "cutlass-fna" backend.
run_persistent_kernel	bool	Whether to use persistent tile scheduling in the forward pass kernel. This only applies to the "blackwell-fna" backend.
kernel_schedule	Optional[str]	Kernel type (Hopper architecture only). Choices are None: pick the default, "non" (non-persistent), "coop" (warp-specialized cooperative), or "pp" (warp-specialized ping-ponging). Refer to Hopper FMHA/FNA backend for more information.
torch_compile	bool	Applies only to the "flex-fna" backend. Whether or not to JIT compile the attention kernel. Due to this being an experimental feature in PyTorch, we do not recommend it, and it is guarded by context flags. Read more in Flex Attention + torch.compile.
attention_kwargs	Optional[Dict]	arguments to the attention operator, if used to implement neighborhood cross-attention, or self attention as a fast path for neighborhood attention. If additional_{keys,values} are specified, NATTEN usually performs a separate cross-attention using our attention operator, and merges the results. If for a given use case, the neighborhood attention problem is equivalent to self attention (not causal along any dims, kernel_size == (X, Y)), NATTEN will also attempt to directly use attention. You can override arguments to attention by passing a dictionary here. Example out = na2d( q, k, v, kernel_size=kernel_size, ..., attention_kwargs={ "backend": "blackwell-fmha", "run_persistent_kernel": True, } )

Returns:

Name	Type	Description
output	Tensor	5-D output tensor, with the heads last layout ([batch, X, Y, heads, head_dim]).

natten.na3d

na3d(
    query,
    key,
    value,
    kernel_size,
    stride=1,
    dilation=1,
    is_causal=False,
    scale=None,
    additional_keys=None,
    additional_values=None,
    attention_kwargs=None,
    backend=None,
    q_tile_shape=None,
    kv_tile_shape=None,
    backward_q_tile_shape=None,
    backward_kv_tile_shape=None,
    backward_kv_splits=None,
    backward_use_pt_reduction=False,
    run_persistent_kernel=True,
    kernel_schedule=None,
    torch_compile=False,
)

Computes 3-D neighborhood attention.

Parameters:

Name	Type	Description	Default
query	Tensor	3-D query tensor, with the heads last layout: [batch, X, Y, Z, heads, head_dim], where token layout shape (feature map shape) is (X, Y, Z).	required
key	Tensor	3-D key tensor, with the heads last layout: [batch, X, Y, Z, heads, head_dim], where token layout shape (feature map shape) is (X, Y, Z).	required
value	Tensor	3-D value tensor, with the heads last layout: [batch, X, Y, Z, heads, head_dim], where token layout shape (feature map shape) is (X, Y, Z).	required
kernel_size	Tuple[int, int, int] | int	Neighborhood window (kernel) size/shape. If an integer, it will be repeated for all 3 dimensions. For example kernel_size=3 is reinterpreted as kernel_size=(3, 3, 3). Note kernel_size must be smaller than or equal to token layout shape ((X, Y, Z)) along every dimension.	required
stride	Tuple[int, int, int] | int	Sliding window step size/shape. Defaults to 1 (standard sliding window). If an integer, it will be repeated for all 3 dimensions. For example stride=2 is reinterpreted as stride=(2, 2, 2). Note stride must be smaller than or equal to kernel_size along every dimension. When stride == kernel_size, there will be no overlap between sliding windows, which is equivalent to blocked attention (a.k.a. window self attention).	1
dilation	Tuple[int, int, int] | int	Dilation step size/shape. Defaults to 1 (standard sliding window). If an integer, it will be repeated for all 3 dimensions. For example dilation=4 is reinterpreted as dilation=(4, 4, 4). Note The product of dilation and kernel_size must be smaller than or equal to token layout shape ((X, Y, Z)) along every dimension.	1
is_causal	Tuple[bool, bool, bool] | bool	Toggle causal masking. Defaults to False (bi-directional). If a boolean, it will be repeated for all 3 dimensions. For example is_causal=True is reinterpreted as is_causal=(True, True, True).	False
scale	float	Attention scale. Defaults to head_dim ** -0.5.	None
additional_keys	Optional[Tensor]	None or 4-D key tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to key tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens.	None
additional_values	Optional[Tensor]	None or 4-D value tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim]), corresponding to value tokens from some additional context. Used when performing neighborhood cross-attention, where query tokens attend to their neighborhood, as well as some fixed additional set of tokens. Note additional_keys and additional_values must both either be Tensors, or both Nones, and must match in shape.	None

Other Parameters:

Name	Type	Description
backend	str	Backend implementation to run with. Choices are: None (pick the best available one), "cutlass-fna", "hopper-fna", "blackwell-fna", "flex-fna". Refer to backends for more information.
q_tile_shape	Tuple[int, int, int]	3-D Tile shape for the query token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
kv_tile_shape	Tuple[int, int, int]	3-D Tile shape for the key-value token layout in the forward pass kernel. You can use profiler to find valid choices for your use case, and search for the best combination.
backward_q_tile_shape	Tuple[int, int, int]	3-D Tile shape for the query token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_tile_shape	Tuple[int, int, int]	3-D Tile shape for the key/value token layout in the backward pass kernel. This is ignored by "flex-fna". You can use profiler to find valid choices for your use case, and search for the best combination.
backward_kv_splits	Tuple[int, int, int]	Number of key/value tiles allowed to work in parallel in the backward pass kernel. Like tile shapes, this is a tuple and not an integer for neighborhood attention operations, and the size of the tuple corresponds to the number of dimensions / rank of the layout of tokens. This is only respected by the "cutlass-fna" backend, and only when KV parallelism is enabled.
backward_use_pt_reduction	bool	Whether to use PyTorch eager for computing the dO * O product required by the backward pass, over the CUTLASS kernel. This only applies to the "cutlass-fna" backend.
run_persistent_kernel	bool	Whether to use persistent tile scheduling in the forward pass kernel. This only applies to the "blackwell-fna" backend.
kernel_schedule	Optional[str]	Kernel type (Hopper architecture only). Choices are None: pick the default, "non" (non-persistent), "coop" (warp-specialized cooperative), or "pp" (warp-specialized ping-ponging). Refer to Hopper FMHA/FNA backend for more information.
torch_compile	bool	Applies only to the "flex-fna" backend. Whether or not to JIT compile the attention kernel. Due to this being an experimental feature in PyTorch, we do not recommend it, and it is guarded by context flags. Read more in Flex Attention + torch.compile.
attention_kwargs	Optional[Dict]	arguments to the attention operator, if used to implement neighborhood cross-attention, or self attention as a fast path for neighborhood attention. If additional_{keys,values} are specified, NATTEN usually performs a separate cross-attention using our attention operator, and merges the results. If for a given use case, the neighborhood attention problem is equivalent to self attention (not causal along any dims, kernel_size == (X, Y, Z)), NATTEN will also attempt to directly use attention. You can override arguments to attention by passing a dictionary here. Example out = na3d( q, k, v, kernel_size=kernel_size, ..., attention_kwargs={ "backend": "blackwell-fmha", "run_persistent_kernel": True, } )

Returns:

Name	Type	Description
output	Tensor	6-D output tensor, with the heads last layout ([batch, X, Y, Z, heads, head_dim]).

Standard Attention

natten.attention

attention(
    query,
    key,
    value,
    is_causal=False,
    scale=None,
    seqlens_Q=None,
    seqlens_KV=None,
    cumulative_seqlen_Q=None,
    cumulative_seqlen_KV=None,
    max_seqlen_Q=None,
    max_seqlen_KV=None,
    backend=None,
    q_tile_size=None,
    kv_tile_size=None,
    backward_q_tile_size=None,
    backward_kv_tile_size=None,
    backward_kv_splits=None,
    backward_use_pt_reduction=False,
    run_persistent_kernel=True,
    kernel_schedule=None,
    torch_compile=False,
    return_lse=False,
)

Runs standard dot product attention.

This operation is used to implement neighborhood cross attention, in which we allow every token to interact with some additional context (additional_keys and additional_values tensors in na1d, na2d, and na3d). This operator is also used as a fast path for cases where neighborhood attention is equivalent to self attention (not causal along any dims, and kernel_size is equal to the number of input tokens).

This operation does not call into PyTorch's SDPA, and only runs one of the NATTEN backends (cutlass-fmha, hopper-fmha, blackwell-fmha, flex-fmha). Reasons for that include being able to control performance-related arguments, return logsumexp, and more. For more information refer to backends.

Causal mask, and Variable length (varlen) Attention are also supported in some backends (cutlass-fmha and blackwell-fmha).

Varlen Attention is only supported for the sequence-packed layout: QKV tensors have batch size 1, and tokens from different batches are concatenated without any padding along the sequence dimension. Sequence lengths for different batches can be provided in two ways: 1. seqlens_Q and seqlens_KV (less efficient): only provide the sequence lengths as integer tensors (must be on the same device as QKV), and NATTEN will compute cumulative and maximum sequence lengths on each call. 2. cumulative_seqlen_{Q,KV} and max_seqlen_{Q,KV} (more efficient): compute cumulative and maximum sequence lengths. cumulative_seqlen_{Q,KV} are integer tensors on the same device as QKV containing the cumulative sum of seqlens_{Q,KV}, with an additional 0 element in the beginning, therefore sized batch+1. max_seqlen_{Q,KV} are integers (not Tensors) that represent the maximum sequence lengths for Q and KV among all sequence batches. You can use natten.utils.varlen.generate_varlen_parameters to generate these parameters:

from natten.utils.varlen import generate_varlen_parameters
(
    cumulative_seqlen_Q,
    cumulative_seqlen_KV,
    max_seqlen_Q,
    max_seqlen_KV,
) = generate_varlen_parameters(q, k, v, seqlens_Q, seqlens_KV)

Parameters:

Name	Type	Description	Default
query	Tensor	4-D query tensor, with the heads last layout ([batch, seqlen, heads, head_dim])	required
key	Tensor	4-D key tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim])	required
value	Tensor	4-D value tensor, with the heads last layout ([batch, seqlen_kv, heads, head_dim_v])	required
is_causal	bool	Toggle causal masking. Defaults to False (bi-directional).	False
scale	float	Attention scale. Defaults to head_dim ** -0.5.	None
seqlens_Q	Optional[Tensor]	(varlen) Optional 1-D tensor with size batch indicating the number of query tokens in each batch. Must be passed together with seqlens_KV.	None
seqlens_KV	Optional[Tensor]	(varlen) Optional 1-D tensor with size batch indicating the number of key/value tokens in each batch. Must be passed together with seqlens_Q.	None
cumulative_seqlen_Q	Optional[Tensor]	(varlen) Optional 1-D tensor with size batch + 1 indicating the cumulative sum of number of query tokens in each batch, with an additional 0 element in the beginning. Must be passed together with cumulative_seqlen_KV and max_seqlen_{Q,KV}.	None
cumulative_seqlen_KV	Optional[Tensor]	(varlen) Optional 1-D tensor with size batch + 1 indicating the cumulative sum of number of key/value tokens in each batch, with an additional 0 element in the beginning. Must be passed together with cumulative_seqlen_Q and max_seqlen_{Q,KV}.	None
max_seqlen_Q	Optional[int]	(varlen) Optional integer indicating the maximum query sequence length in all batches. Must be passed together with cumulative_seqlen_{Q,KV} and max_seqlen_KV.	None
max_seqlen_KV	Optional[int]	(varlen) Optional integer indicating the maximum key/value sequence length in all batches. Must be passed together with cumulative_seqlen_{Q,KV} and max_seqlen_Q.	None

Other Parameters:

Name	Type	Description
backend	str	Backend implementation to run with. Choices are: None (pick the best available one), "cutlass-fmha", "hopper-fmha", "blackwell-fmha", "flex-fmha". Refer to backends for more information.
q_tile_size	int	Tile size along query sequence length in the forward pass kernel. You can use profiler to find valid choices for your use case.
kv_tile_size	int	Tile size along key/value sequence length in the forward pass kernel. You can use profiler to find valid choices for your use case.
backward_q_tile_size	int	Tile size along query sequence length in the backward pass kernel. This is ignored by "flex-fmha". You can use profiler to find valid choices for your use case.
backward_kv_tile_size	int	Tile size along key/value sequence length in the backward pass kernel. This is ignored by "flex-fmha". You can use profiler to find valid choices for your use case.
backward_kv_splits	int	Number of key/value tiles allowed to work in parallel in the backward pass kernel. This is only respected by the "cutlass-fmha" backend, only when KV parallelism is enabled.
backward_use_pt_reduction	bool	Whether to use PyTorch eager for computing the dO * O product required by the backward pass, over the CUTLASS kernel. This only applies to the "cutlass-fmha" backend.
run_persistent_kernel	bool	Whether to use persistent tile scheduling in the forward pass kernel. This only applies to the "blackwell-fmha" backend.
kernel_schedule	Optional[str]	Kernel type (Hopper architecture only). Choices are None: pick the default, "non" (non-persistent), "coop" (warp-specialized cooperative), or "pp" (warp-specialized ping-ponging). Refer to Hopper FMHA/FNA backend for more information.
torch_compile	bool	Applies only to the "flex-fmha" backend. Whether or not to JIT compile the attention kernel. Due to this being an experimental feature in PyTorch, we do not recommend it, and it is guarded by context flags. Read more in Flex Attention + torch.compile.
return_lse	bool	Whether or not to return the logsumexp tensor. logsumexp can be used in the backward pass, and for attention merging.

Returns:

Name	Type	Description
output	Tensor	4-D output tensor, with the heads last layout ([batch, seqlen, heads, head_dim_v]).
logsumexp	Tensor	only returned when return_lse=True. 3-D logsumexp tensor, with the heads last layout ([batch, seqlen, heads]).

natten.merge_attentions

merge_attentions(outputs, lse_tensors, torch_compile=True)

Takes multiple attention outputs originating from the same query tensor, and their corresponding logsumexps, and merges them as if their context (key/value pair) had been concatenated.

This operation is used to implement cross-neighborhood attention, and can also be used for distributed setups, such as context-parallelism.

This operation also attempts to use torch.compile to fuse the elementwise operations. This can be disabled by passing torch_compile=False.

Parameters:

Name	Type	Description	Default
outputs	List[Tensor]	List of 4-D attention output tensors, with the heads last layout ([batch, seqlen, heads, head_dim])	required
lse_tensors	List[Tensor]	List of 3-D logsumexp tensors, with the heads last layout ([batch, seqlen, heads])	required
torch_compile	bool	Attempt to use torch.compile to fuse the underlying elementwise operations.	True

Returns:

Name	Type	Description
output	Tensor	merged attention output.