Paper address: BiFormer: Vision Transformer with Bi-Level Routing Attention
Code address: https://github.com/rayleizhu/BiFormer
Attention is an extremely important module in the vision transformer, but it has a very big shortcoming: the amount of calculation is too large.
BiFormer proposed Bi-Level Routing Attention. When calculating Attention, only the most important tokens are focused on, thereby reducing the amount of calculation.
一、Bi-Level Routing Attention
The figure below shows the areas that multiple different Attention modules focus on. (a) is the original attention, and the others are sparse Attention structures. Bi-Level Routing Attention is shown in (f) below.
Different from other sparse Attention structures, Bi-Level Routing Attention first divides the feature map into different areas (the area size is SxS). Each area is linearly mapped to obtain QKV, and then QK is within the window of each SxS. Taking the average as the token of the area (refer to the code), we get (r represents region, that is, the window of SxS), which
is obtained by matrix operation
, as follows:
After obtaining the adjacency matrix , take the k token indexes with the highest correlation
, so that you know which k windows each window has a higher correlation with.
After getting it , use the gather operation to get
the sum
:
Finally calculate Attention:
The calculation process of Bi-Level Routing Attention is shown in the figure below, where k is the parameter set when calculating the correlation index.
2. Code
The code for Bi-Level Routing Attention is as follows:
"""
Core of BiFormer, Bi-Level Routing Attention.
To be refactored.
author: ZHU Lei
github: https://github.com/rayleizhu
email: [email protected]
This source code is licensed under the license found in the
LICENSE file in the root directory of this source tree.
"""
from typing import Tuple
import torch
import torch.nn as nn
import torch.nn.functional as F
from einops import rearrange
from torch import Tensor
class TopkRouting(nn.Module):
"""
differentiable topk routing with scaling
Args:
qk_dim: int, feature dimension of query and key
topk: int, the 'topk'
qk_scale: int or None, temperature (multiply) of softmax activation
with_param: bool, wether inorporate learnable params in routing unit
diff_routing: bool, wether make routing differentiable
soft_routing: bool, wether make output value multiplied by routing weights
"""
def __init__(self, qk_dim, topk=4, qk_scale=None, param_routing=False, diff_routing=False):
super().__init__()
self.topk = topk
self.qk_dim = qk_dim
self.scale = qk_scale or qk_dim ** -0.5
self.diff_routing = diff_routing
# TODO: norm layer before/after linear?
self.emb = nn.Linear(qk_dim, qk_dim) if param_routing else nn.Identity()
# routing activation
self.routing_act = nn.Softmax(dim=-1)
def forward(self, query:Tensor, key:Tensor)->Tuple[Tensor]:
"""
Args:
q, k: (n, p^2, c) tensor
Return:
r_weight, topk_index: (n, p^2, topk) tensor
"""
if not self.diff_routing:
query, key = query.detach(), key.detach()
query_hat, key_hat = self.emb(query), self.emb(key) # per-window pooling -> (n, p^2, c)
attn_logit = (query_hat*self.scale) @ key_hat.transpose(-2, -1) # (n, p^2, p^2)
topk_attn_logit, topk_index = torch.topk(attn_logit, k=self.topk, dim=-1) # (n, p^2, k), (n, p^2, k)
r_weight = self.routing_act(topk_attn_logit) # (n, p^2, k)
return r_weight, topk_index
class KVGather(nn.Module):
def __init__(self, mul_weight='none'):
super().__init__()
assert mul_weight in ['none', 'soft', 'hard']
self.mul_weight = mul_weight
def forward(self, r_idx:Tensor, r_weight:Tensor, kv:Tensor):
"""
r_idx: (n, p^2, topk) tensor
r_weight: (n, p^2, topk) tensor
kv: (n, p^2, w^2, c_kq+c_v)
Return:
(n, p^2, topk, w^2, c_kq+c_v) tensor
"""
# select kv according to routing index
n, p2, w2, c_kv = kv.size()
topk = r_idx.size(-1)
# print(r_idx.size(), r_weight.size())
# FIXME: gather consumes much memory (topk times redundancy), write cuda kernel?
topk_kv = torch.gather(kv.view(n, 1, p2, w2, c_kv).expand(-1, p2, -1, -1, -1), # (n, p^2, p^2, w^2, c_kv) without mem cpy
dim=2,
index=r_idx.view(n, p2, topk, 1, 1).expand(-1, -1, -1, w2, c_kv) # (n, p^2, k, w^2, c_kv)
)
if self.mul_weight == 'soft':
topk_kv = r_weight.view(n, p2, topk, 1, 1) * topk_kv # (n, p^2, k, w^2, c_kv)
elif self.mul_weight == 'hard':
raise NotImplementedError('differentiable hard routing TBA')
# else: #'none'
# topk_kv = topk_kv # do nothing
return topk_kv
class QKVLinear(nn.Module):
def __init__(self, dim, qk_dim, bias=True):
super().__init__()
self.dim = dim
self.qk_dim = qk_dim
self.qkv = nn.Linear(dim, qk_dim + qk_dim + dim, bias=bias)
def forward(self, x):
q, kv = self.qkv(x).split([self.qk_dim, self.qk_dim+self.dim], dim=-1)
return q, kv
# q, k, v = self.qkv(x).split([self.qk_dim, self.qk_dim, self.dim], dim=-1)
# return q, k, v
class BiLevelRoutingAttention(nn.Module):
"""
n_win: number of windows in one side (so the actual number of windows is n_win*n_win)
kv_per_win: for kv_downsample_mode='ada_xxxpool' only, number of key/values per window. Similar to n_win, the actual number is kv_per_win*kv_per_win.
topk: topk for window filtering
param_attention: 'qkvo'-linear for q,k,v and o, 'none': param free attention
param_routing: extra linear for routing
diff_routing: wether to set routing differentiable
soft_routing: wether to multiply soft routing weights
"""
def __init__(self, dim, num_heads=8, n_win=7, qk_dim=None, qk_scale=None,
kv_per_win=4, kv_downsample_ratio=4, kv_downsample_kernel=None, kv_downsample_mode='identity',
topk=4, param_attention="qkvo", param_routing=False, diff_routing=False, soft_routing=False, side_dwconv=3,
auto_pad=False):
super().__init__()
# local attention setting
self.dim = dim
self.n_win = n_win # Wh, Ww
self.num_heads = num_heads
self.qk_dim = qk_dim or dim
assert self.qk_dim % num_heads == 0 and self.dim % num_heads==0, 'qk_dim and dim must be divisible by num_heads!'
self.scale = qk_scale or self.qk_dim ** -0.5
################side_dwconv (i.e. LCE in ShuntedTransformer)###########
self.lepe = nn.Conv2d(dim, dim, kernel_size=side_dwconv, stride=1, padding=side_dwconv//2, groups=dim) if side_dwconv > 0 else \
lambda x: torch.zeros_like(x)
################ global routing setting #################
self.topk = topk
self.param_routing = param_routing
self.diff_routing = diff_routing
self.soft_routing = soft_routing
# router
assert not (self.param_routing and not self.diff_routing) # cannot be with_param=True and diff_routing=False
self.router = TopkRouting(qk_dim=self.qk_dim,
qk_scale=self.scale,
topk=self.topk,
diff_routing=self.diff_routing,
param_routing=self.param_routing)
if self.soft_routing: # soft routing, always diffrentiable (if no detach)
mul_weight = 'soft'
elif self.diff_routing: # hard differentiable routing
mul_weight = 'hard'
else: # hard non-differentiable routing
mul_weight = 'none'
self.kv_gather = KVGather(mul_weight=mul_weight)
# qkv mapping (shared by both global routing and local attention)
self.param_attention = param_attention
if self.param_attention == 'qkvo':
self.qkv = QKVLinear(self.dim, self.qk_dim)
self.wo = nn.Linear(dim, dim)
elif self.param_attention == 'qkv':
self.qkv = QKVLinear(self.dim, self.qk_dim)
self.wo = nn.Identity()
else:
raise ValueError(f'param_attention mode {self.param_attention} is not surpported!')
self.kv_downsample_mode = kv_downsample_mode
self.kv_per_win = kv_per_win
self.kv_downsample_ratio = kv_downsample_ratio
self.kv_downsample_kenel = kv_downsample_kernel
if self.kv_downsample_mode == 'ada_avgpool':
assert self.kv_per_win is not None
self.kv_down = nn.AdaptiveAvgPool2d(self.kv_per_win)
elif self.kv_downsample_mode == 'ada_maxpool':
assert self.kv_per_win is not None
self.kv_down = nn.AdaptiveMaxPool2d(self.kv_per_win)
elif self.kv_downsample_mode == 'maxpool':
assert self.kv_downsample_ratio is not None
self.kv_down = nn.MaxPool2d(self.kv_downsample_ratio) if self.kv_downsample_ratio > 1 else nn.Identity()
elif self.kv_downsample_mode == 'avgpool':
assert self.kv_downsample_ratio is not None
self.kv_down = nn.AvgPool2d(self.kv_downsample_ratio) if self.kv_downsample_ratio > 1 else nn.Identity()
elif self.kv_downsample_mode == 'identity': # no kv downsampling
self.kv_down = nn.Identity()
elif self.kv_downsample_mode == 'fracpool':
# assert self.kv_downsample_ratio is not None
# assert self.kv_downsample_kenel is not None
# TODO: fracpool
# 1. kernel size should be input size dependent
# 2. there is a random factor, need to avoid independent sampling for k and v
raise NotImplementedError('fracpool policy is not implemented yet!')
elif kv_downsample_mode == 'conv':
# TODO: need to consider the case where k != v so that need two downsample modules
raise NotImplementedError('conv policy is not implemented yet!')
else:
raise ValueError(f'kv_down_sample_mode {self.kv_downsaple_mode} is not surpported!')
# softmax for local attention
self.attn_act = nn.Softmax(dim=-1)
self.auto_pad=auto_pad
def forward(self, x, ret_attn_mask=False):
"""
x: NHWC tensor
Return:
NHWC tensor
"""
# NOTE: use padding for semantic segmentation
###################################################
if self.auto_pad:
N, H_in, W_in, C = x.size()
pad_l = pad_t = 0
pad_r = (self.n_win - W_in % self.n_win) % self.n_win
pad_b = (self.n_win - H_in % self.n_win) % self.n_win
x = F.pad(x, (0, 0, # dim=-1
pad_l, pad_r, # dim=-2
pad_t, pad_b)) # dim=-3
_, H, W, _ = x.size() # padded size
else:
N, H, W, C = x.size()
assert H%self.n_win == 0 and W%self.n_win == 0 #
###################################################
# patchify, (n, p^2, w, w, c), keep 2d window as we need 2d pooling to reduce kv size
x = rearrange(x, "n (j h) (i w) c -> n (j i) h w c", j=self.n_win, i=self.n_win)
#################qkv projection###################
# q: (n, p^2, w, w, c_qk)
# kv: (n, p^2, w, w, c_qk+c_v)
# NOTE: separte kv if there were memory leak issue caused by gather
q, kv = self.qkv(x)
# pixel-wise qkv
# q_pix: (n, p^2, w^2, c_qk)
# kv_pix: (n, p^2, h_kv*w_kv, c_qk+c_v)
q_pix = rearrange(q, 'n p2 h w c -> n p2 (h w) c')
kv_pix = self.kv_down(rearrange(kv, 'n p2 h w c -> (n p2) c h w'))
kv_pix = rearrange(kv_pix, '(n j i) c h w -> n (j i) (h w) c', j=self.n_win, i=self.n_win)
q_win, k_win = q.mean([2, 3]), kv[..., 0:self.qk_dim].mean([2, 3]) # window-wise qk, (n, p^2, c_qk), (n, p^2, c_qk)
##################side_dwconv(lepe)##################
# NOTE: call contiguous to avoid gradient warning when using ddp
lepe = self.lepe(rearrange(kv[..., self.qk_dim:], 'n (j i) h w c -> n c (j h) (i w)', j=self.n_win, i=self.n_win).contiguous())
lepe = rearrange(lepe, 'n c (j h) (i w) -> n (j h) (i w) c', j=self.n_win, i=self.n_win)
############ gather q dependent k/v #################
r_weight, r_idx = self.router(q_win, k_win) # both are (n, p^2, topk) tensors
kv_pix_sel = self.kv_gather(r_idx=r_idx, r_weight=r_weight, kv=kv_pix) #(n, p^2, topk, h_kv*w_kv, c_qk+c_v)
k_pix_sel, v_pix_sel = kv_pix_sel.split([self.qk_dim, self.dim], dim=-1)
# kv_pix_sel: (n, p^2, topk, h_kv*w_kv, c_qk)
# v_pix_sel: (n, p^2, topk, h_kv*w_kv, c_v)
######### do attention as normal ####################
k_pix_sel = rearrange(k_pix_sel, 'n p2 k w2 (m c) -> (n p2) m c (k w2)', m=self.num_heads) # flatten to BMLC, (n*p^2, m, topk*h_kv*w_kv, c_kq//m) transpose here?
v_pix_sel = rearrange(v_pix_sel, 'n p2 k w2 (m c) -> (n p2) m (k w2) c', m=self.num_heads) # flatten to BMLC, (n*p^2, m, topk*h_kv*w_kv, c_v//m)
q_pix = rearrange(q_pix, 'n p2 w2 (m c) -> (n p2) m w2 c', m=self.num_heads) # to BMLC tensor (n*p^2, m, w^2, c_qk//m)
# param-free multihead attention
attn_weight = (q_pix * self.scale) @ k_pix_sel # (n*p^2, m, w^2, c) @ (n*p^2, m, c, topk*h_kv*w_kv) -> (n*p^2, m, w^2, topk*h_kv*w_kv)
attn_weight = self.attn_act(attn_weight)
out = attn_weight @ v_pix_sel # (n*p^2, m, w^2, topk*h_kv*w_kv) @ (n*p^2, m, topk*h_kv*w_kv, c) -> (n*p^2, m, w^2, c)
out = rearrange(out, '(n j i) m (h w) c -> n (j h) (i w) (m c)', j=self.n_win, i=self.n_win,
h=H//self.n_win, w=W//self.n_win)
out = out + lepe
# output linear
out = self.wo(out)
# NOTE: use padding for semantic segmentation
# crop padded region
if self.auto_pad and (pad_r > 0 or pad_b > 0):
out = out[:, :H_in, :W_in, :].contiguous()
if ret_attn_mask:
return out, r_weight, r_idx, attn_weight
else:
return out