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7375975 | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 | from collections import namedtuple
from functools import wraps
from packaging import version
import torch
from torch import nn, einsum
import torch.nn.functional as F
from einops import rearrange
# constants
Config = namedtuple('EfficientAttentionConfig', ['enable_flash', 'enable_math', 'enable_mem_efficient'])
# helpers
def exists(val):
return val is not None
def once(fn):
called = False
@wraps(fn)
def inner(x):
nonlocal called
if called:
return
called = True
return fn(x)
return inner
print_once = once(print)
# main class
class Attend(nn.Module):
def __init__(
self,
dropout = 0.,
causal = False,
use_flash = False
):
super().__init__()
self.dropout = dropout
self.attn_dropout = nn.Dropout(dropout)
self.causal = causal
self.register_buffer("mask", None, persistent=False)
self.use_flash = use_flash
assert not (use_flash and version.parse(torch.__version__) < version.parse('2.0.0')), 'in order to use flash attention, you must be using pytorch 2.0 or above'
# determine efficient attention configs for cuda and cpu
self.cpu_config = Config(True, True, True)
self.cuda_config = None
if not torch.cuda.is_available() or not use_flash:
return
device_properties = torch.cuda.get_device_properties(torch.device('cuda'))
if device_properties.major == 8 and device_properties.minor == 0:
print_once('A100 GPU detected, using flash attention if input tensor is on cuda')
self.cuda_config = Config(True, False, False)
else:
print_once('Non-A100 GPU detected, using math or mem efficient attention if input tensor is on cuda')
self.cuda_config = Config(False, True, True)
def get_mask(self, n, device):
if exists(self.mask) and self.mask.shape[-1] >= n:
return self.mask[:n, :n]
mask = torch.ones((n, n), device=device, dtype=torch.bool).triu(1)
self.register_buffer("mask", mask, persistent=False)
return mask
def flash_attn(self, q, k, v, mask = None):
_, heads, q_len, _, k_len, is_cuda = *q.shape, k.shape[-2], q.is_cuda
# Recommended for multi-query single-key-value attention by Tri Dao
# kv shape torch.Size([1, 512, 64]) -> torch.Size([1, 8, 512, 64])
if k.ndim == 3:
k = rearrange(k, 'b ... -> b 1 ...').expand_as(q)
if v.ndim == 3:
v = rearrange(v, 'b ... -> b 1 ...').expand_as(q)
# Check if mask exists and expand to compatible shape
# The mask is B L, so it would have to be expanded to B H N L
if exists(mask):
mask = rearrange(mask, 'b j -> b 1 1 j')
mask = mask.expand(-1, heads, q_len, -1)
# Check if there is a compatible device for flash attention
config = self.cuda_config if is_cuda else self.cpu_config
# pytorch 2.0 flash attn: q, k, v, mask, dropout, causal, softmax_scale
with torch.backends.cuda.sdp_kernel(**config._asdict()):
out = F.scaled_dot_product_attention(
q, k, v,
attn_mask = mask,
dropout_p = self.dropout if self.training else 0.,
is_causal = self.causal
)
return out
def forward(self, q, k, v, mask = None):
"""
einstein notation
b - batch
h - heads
n, i, j - sequence length (base sequence length, source, target)
d - feature dimension
"""
n, device = q.shape[-2], q.device
scale = q.shape[-1] ** -0.5
if self.use_flash:
return self.flash_attn(q, k, v, mask = mask)
kv_einsum_eq = 'b j d' if k.ndim == 3 else 'b h j d'
# similarity
sim = einsum(f"b h i d, {kv_einsum_eq} -> b h i j", q, k) * scale
# key padding mask
if exists(mask):
mask = rearrange(mask, 'b j -> b 1 1 j')
sim = sim.masked_fill(~mask, -torch.finfo(sim.dtype).max)
# causal mask
if self.causal:
causal_mask = self.get_mask(n, device)
sim = sim.masked_fill(causal_mask, -torch.finfo(sim.dtype).max)
# attention
attn = sim.softmax(dim=-1)
attn = self.attn_dropout(attn)
# aggregate values
out = einsum(f"b h i j, {kv_einsum_eq} -> b h i d", attn, v)
return out
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