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import importlib.metadata
import math
import torch
import torch.nn as nn
import torch.nn.functional as F
try:
import flash_attn
from flash_attn.flash_attn_interface import _flash_attn_forward
from flash_attn.flash_attn_interface import flash_attn_varlen_func
except ImportError:
flash_attn = None
flash_attn_varlen_func = None
_flash_attn_forward = None
MEMORY_LAYOUT = {
"flash": (
lambda x: x.view(x.shape[0] * x.shape[1], *x.shape[2:]),
lambda x: x,
),
"torch": (
lambda x: x.transpose(1, 2),
lambda x: x.transpose(1, 2),
),
"vanilla": (
lambda x: x.transpose(1, 2),
lambda x: x.transpose(1, 2),
),
}
def get_cu_seqlens(text_mask, img_len):
"""Calculate cu_seqlens_q, cu_seqlens_kv using text_mask and img_len
Args:
text_mask (torch.Tensor): the mask of text
img_len (int): the length of image
Returns:
torch.Tensor: the calculated cu_seqlens for flash attention
"""
batch_size = text_mask.shape[0]
text_len = text_mask.sum(dim=1)
max_len = text_mask.shape[1] + img_len
cu_seqlens = torch.zeros([2 * batch_size + 1], dtype=torch.int32, device="cuda")
for i in range(batch_size):
s = text_len[i] + img_len
s1 = i * max_len + s
s2 = (i + 1) * max_len
cu_seqlens[2 * i + 1] = s1
cu_seqlens[2 * i + 2] = s2
return cu_seqlens
def attention(
q,
k,
v,
mode="flash",
drop_rate=0,
attn_mask=None,
causal=False,
cu_seqlens_q=None,
cu_seqlens_kv=None,
max_seqlen_q=None,
max_seqlen_kv=None,
batch_size=1,
):
"""
Perform QKV self attention.
Args:
q (torch.Tensor): Query tensor with shape [b, s, a, d], where a is the number of heads.
k (torch.Tensor): Key tensor with shape [b, s1, a, d]
v (torch.Tensor): Value tensor with shape [b, s1, a, d]
mode (str): Attention mode. Choose from 'self_flash', 'cross_flash', 'torch', and 'vanilla'.
drop_rate (float): Dropout rate in attention map. (default: 0)
attn_mask (torch.Tensor): Attention mask with shape [b, s1] (cross_attn), or [b, a, s, s1] (torch or vanilla).
(default: None)
causal (bool): Whether to use causal attention. (default: False)
cu_seqlens_q (torch.Tensor): dtype torch.int32. The cumulative sequence lengths of the sequences in the batch,
used to index into q.
cu_seqlens_kv (torch.Tensor): dtype torch.int32. The cumulative sequence lengths of the sequences in the batch,
used to index into kv.
max_seqlen_q (int): The maximum sequence length in the batch of q.
max_seqlen_kv (int): The maximum sequence length in the batch of k and v.
Returns:
torch.Tensor: Output tensor after self attention with shape [b, s, ad]
"""
pre_attn_layout, post_attn_layout = MEMORY_LAYOUT[mode]
q = pre_attn_layout(q)
k = pre_attn_layout(k)
v = pre_attn_layout(v)
if mode == "torch":
if attn_mask is not None and attn_mask.dtype != torch.bool:
attn_mask = attn_mask.to(q.dtype)
x = F.scaled_dot_product_attention(
q, k, v, attn_mask=attn_mask, dropout_p=drop_rate, is_causal=causal
)
elif mode == "flash":
x = flash_attn_varlen_func(
q,
k,
v,
cu_seqlens_q,
cu_seqlens_kv,
max_seqlen_q,
max_seqlen_kv,
)
# x with shape [(bxs), a, d]
x = x.view(
batch_size, max_seqlen_q, x.shape[-2], x.shape[-1]
) # reshape x to [b, s, a, d]
elif mode == "vanilla":
scale_factor = 1 / math.sqrt(q.size(-1))
b, a, s, _ = q.shape
s1 = k.size(2)
attn_bias = torch.zeros(b, a, s, s1, dtype=q.dtype, device=q.device)
if causal:
# Only applied to self attention
assert (
attn_mask is None
), "Causal mask and attn_mask cannot be used together"
temp_mask = torch.ones(b, a, s, s, dtype=torch.bool, device=q.device).tril(
diagonal=0
)
attn_bias.masked_fill_(temp_mask.logical_not(), float("-inf"))
attn_bias.to(q.dtype)
if attn_mask is not None:
if attn_mask.dtype == torch.bool:
attn_bias.masked_fill_(attn_mask.logical_not(), float("-inf"))
else:
attn_bias += attn_mask
# TODO: Maybe force q and k to be float32 to avoid numerical overflow
attn = (q @ k.transpose(-2, -1)) * scale_factor
attn += attn_bias
attn = attn.softmax(dim=-1)
attn = torch.dropout(attn, p=drop_rate, train=True)
x = attn @ v
else:
raise NotImplementedError(f"Unsupported attention mode: {mode}")
x = post_attn_layout(x)
b, s, a, d = x.shape
out = x.reshape(b, s, -1)
return out
def parallel_attention(
hybrid_seq_parallel_attn,
q,
k,
v,
img_q_len,
img_kv_len,
cu_seqlens_q,
cu_seqlens_kv
):
attn1 = hybrid_seq_parallel_attn(
None,
q[:, :img_q_len, :, :],
k[:, :img_kv_len, :, :],
v[:, :img_kv_len, :, :],
dropout_p=0.0,
causal=False,
joint_tensor_query=q[:,img_q_len:cu_seqlens_q[1]],
joint_tensor_key=k[:,img_kv_len:cu_seqlens_kv[1]],
joint_tensor_value=v[:,img_kv_len:cu_seqlens_kv[1]],
joint_strategy="rear",
)
if flash_attn.__version__ >= '2.7.0':
attn2, *_ = _flash_attn_forward(
q[:,cu_seqlens_q[1]:],
k[:,cu_seqlens_kv[1]:],
v[:,cu_seqlens_kv[1]:],
dropout_p=0.0,
softmax_scale=q.shape[-1] ** (-0.5),
causal=False,
window_size_left=-1,
window_size_right=-1,
softcap=0.0,
alibi_slopes=None,
return_softmax=False,
)
else:
attn2, *_ = _flash_attn_forward(
q[:,cu_seqlens_q[1]:],
k[:,cu_seqlens_kv[1]:],
v[:,cu_seqlens_kv[1]:],
dropout_p=0.0,
softmax_scale=q.shape[-1] ** (-0.5),
causal=False,
window_size=(-1, -1),
softcap=0.0,
alibi_slopes=None,
return_softmax=False,
)
attn = torch.cat([attn1, attn2], dim=1)
b, s, a, d = attn.shape
attn = attn.reshape(b, s, -1)
return attn
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