Source code for mmseg.models.backbones.mit
# Copyright (c) OpenMMLab. All rights reserved.
import math
import warnings
import torch
import torch.nn as nn
from mmcv.cnn import Conv2d, build_activation_layer, build_norm_layer
from mmcv.cnn.bricks.drop import build_dropout
from mmcv.cnn.bricks.transformer import MultiheadAttention
from mmcv.cnn.utils.weight_init import (constant_init, normal_init,
trunc_normal_init)
from mmcv.runner import BaseModule, ModuleList, Sequential
from ..builder import BACKBONES
from ..utils import PatchEmbed, nchw_to_nlc, nlc_to_nchw
class MixFFN(BaseModule):
"""An implementation of MixFFN of Segformer.
The differences between MixFFN & FFN:
1. Use 1X1 Conv to replace Linear layer.
2. Introduce 3X3 Conv to encode positional information.
Args:
embed_dims (int): The feature dimension. Same as
`MultiheadAttention`. Defaults: 256.
feedforward_channels (int): The hidden dimension of FFNs.
Defaults: 1024.
act_cfg (dict, optional): The activation config for FFNs.
Default: dict(type='ReLU')
ffn_drop (float, optional): Probability of an element to be
zeroed in FFN. Default 0.0.
dropout_layer (obj:`ConfigDict`): The dropout_layer used
when adding the shortcut.
init_cfg (obj:`mmcv.ConfigDict`): The Config for initialization.
Default: None.
"""
def __init__(self,
embed_dims,
feedforward_channels,
act_cfg=dict(type='GELU'),
ffn_drop=0.,
dropout_layer=None,
init_cfg=None):
super(MixFFN, self).__init__(init_cfg)
self.embed_dims = embed_dims
self.feedforward_channels = feedforward_channels
self.act_cfg = act_cfg
self.activate = build_activation_layer(act_cfg)
in_channels = embed_dims
fc1 = Conv2d(
in_channels=in_channels,
out_channels=feedforward_channels,
kernel_size=1,
stride=1,
bias=True)
# 3x3 depth wise conv to provide positional encode information
pe_conv = Conv2d(
in_channels=feedforward_channels,
out_channels=feedforward_channels,
kernel_size=3,
stride=1,
padding=(3 - 1) // 2,
bias=True,
groups=feedforward_channels)
fc2 = Conv2d(
in_channels=feedforward_channels,
out_channels=in_channels,
kernel_size=1,
stride=1,
bias=True)
drop = nn.Dropout(ffn_drop)
layers = [fc1, pe_conv, self.activate, drop, fc2, drop]
self.layers = Sequential(*layers)
self.dropout_layer = build_dropout(
dropout_layer) if dropout_layer else torch.nn.Identity()
def forward(self, x, hw_shape, identity=None):
out = nlc_to_nchw(x, hw_shape)
out = self.layers(out)
out = nchw_to_nlc(out)
if identity is None:
identity = x
return identity + self.dropout_layer(out)
class EfficientMultiheadAttention(MultiheadAttention):
"""An implementation of Efficient Multi-head Attention of Segformer.
This module is modified from MultiheadAttention which is a module from
mmcv.cnn.bricks.transformer.
Args:
embed_dims (int): The embedding dimension.
num_heads (int): Parallel attention heads.
attn_drop (float): A Dropout layer on attn_output_weights.
Default: 0.0.
proj_drop (float): A Dropout layer after `nn.MultiheadAttention`.
Default: 0.0.
dropout_layer (obj:`ConfigDict`): The dropout_layer used
when adding the shortcut. Default: None.
init_cfg (obj:`mmcv.ConfigDict`): The Config for initialization.
Default: None.
batch_first (bool): Key, Query and Value are shape of
(batch, n, embed_dim)
or (n, batch, embed_dim). Default: False.
qkv_bias (bool): enable bias for qkv if True. Default True.
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='LN').
sr_ratio (int): The ratio of spatial reduction of Efficient Multi-head
Attention of Segformer. Default: 1.
"""
def __init__(self,
embed_dims,
num_heads,
attn_drop=0.,
proj_drop=0.,
dropout_layer=None,
init_cfg=None,
batch_first=True,
qkv_bias=False,
norm_cfg=dict(type='LN'),
sr_ratio=1):
super().__init__(
embed_dims,
num_heads,
attn_drop,
proj_drop,
dropout_layer=dropout_layer,
init_cfg=init_cfg,
batch_first=batch_first,
bias=qkv_bias)
self.sr_ratio = sr_ratio
if sr_ratio > 1:
self.sr = Conv2d(
in_channels=embed_dims,
out_channels=embed_dims,
kernel_size=sr_ratio,
stride=sr_ratio)
# The ret[0] of build_norm_layer is norm name.
self.norm = build_norm_layer(norm_cfg, embed_dims)[1]
# handle the BC-breaking from https://github.com/open-mmlab/mmcv/pull/1418 # noqa
from mmseg import digit_version, mmcv_version
if mmcv_version < digit_version('1.3.17'):
warnings.warn('The legacy version of forward function in'
'EfficientMultiheadAttention is deprecated in'
'mmcv>=1.3.17 and will no longer support in the'
'future. Please upgrade your mmcv.')
self.forward = self.legacy_forward
def forward(self, x, hw_shape, identity=None):
x_q = x
if self.sr_ratio > 1:
x_kv = nlc_to_nchw(x, hw_shape)
x_kv = self.sr(x_kv)
x_kv = nchw_to_nlc(x_kv)
x_kv = self.norm(x_kv)
else:
x_kv = x
if identity is None:
identity = x_q
# Because the dataflow('key', 'query', 'value') of
# ``torch.nn.MultiheadAttention`` is (num_query, batch,
# embed_dims), We should adjust the shape of dataflow from
# batch_first (batch, num_query, embed_dims) to num_query_first
# (num_query ,batch, embed_dims), and recover ``attn_output``
# from num_query_first to batch_first.
if self.batch_first:
x_q = x_q.transpose(0, 1)
x_kv = x_kv.transpose(0, 1)
out = self.attn(query=x_q, key=x_kv, value=x_kv)[0]
if self.batch_first:
out = out.transpose(0, 1)
return identity + self.dropout_layer(self.proj_drop(out))
def legacy_forward(self, x, hw_shape, identity=None):
"""multi head attention forward in mmcv version < 1.3.17."""
x_q = x
if self.sr_ratio > 1:
x_kv = nlc_to_nchw(x, hw_shape)
x_kv = self.sr(x_kv)
x_kv = nchw_to_nlc(x_kv)
x_kv = self.norm(x_kv)
else:
x_kv = x
if identity is None:
identity = x_q
# `need_weights=True` will let nn.MultiHeadAttention
# `return attn_output, attn_output_weights.sum(dim=1) / num_heads`
# The `attn_output_weights.sum(dim=1)` may cause cuda error. So, we set
# `need_weights=False` to ignore `attn_output_weights.sum(dim=1)`.
# This issue - `https://github.com/pytorch/pytorch/issues/37583` report
# the error that large scale tensor sum operation may cause cuda error.
out = self.attn(query=x_q, key=x_kv, value=x_kv, need_weights=False)[0]
return identity + self.dropout_layer(self.proj_drop(out))
class TransformerEncoderLayer(BaseModule):
"""Implements one encoder layer in Segformer.
Args:
embed_dims (int): The feature dimension.
num_heads (int): Parallel attention heads.
feedforward_channels (int): The hidden dimension for FFNs.
drop_rate (float): Probability of an element to be zeroed.
after the feed forward layer. Default 0.0.
attn_drop_rate (float): The drop out rate for attention layer.
Default 0.0.
drop_path_rate (float): stochastic depth rate. Default 0.0.
qkv_bias (bool): enable bias for qkv if True.
Default: True.
act_cfg (dict): The activation config for FFNs.
Default: dict(type='GELU').
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='LN').
batch_first (bool): Key, Query and Value are shape of
(batch, n, embed_dim)
or (n, batch, embed_dim). Default: False.
init_cfg (dict, optional): Initialization config dict.
Default:None.
sr_ratio (int): The ratio of spatial reduction of Efficient Multi-head
Attention of Segformer. Default: 1.
"""
def __init__(self,
embed_dims,
num_heads,
feedforward_channels,
drop_rate=0.,
attn_drop_rate=0.,
drop_path_rate=0.,
qkv_bias=True,
act_cfg=dict(type='GELU'),
norm_cfg=dict(type='LN'),
batch_first=True,
sr_ratio=1):
super(TransformerEncoderLayer, self).__init__()
# The ret[0] of build_norm_layer is norm name.
self.norm1 = build_norm_layer(norm_cfg, embed_dims)[1]
self.attn = EfficientMultiheadAttention(
embed_dims=embed_dims,
num_heads=num_heads,
attn_drop=attn_drop_rate,
proj_drop=drop_rate,
dropout_layer=dict(type='DropPath', drop_prob=drop_path_rate),
batch_first=batch_first,
qkv_bias=qkv_bias,
norm_cfg=norm_cfg,
sr_ratio=sr_ratio)
# The ret[0] of build_norm_layer is norm name.
self.norm2 = build_norm_layer(norm_cfg, embed_dims)[1]
self.ffn = MixFFN(
embed_dims=embed_dims,
feedforward_channels=feedforward_channels,
ffn_drop=drop_rate,
dropout_layer=dict(type='DropPath', drop_prob=drop_path_rate),
act_cfg=act_cfg)
def forward(self, x, hw_shape):
x = self.attn(self.norm1(x), hw_shape, identity=x)
x = self.ffn(self.norm2(x), hw_shape, identity=x)
return x
[docs]@BACKBONES.register_module()
class MixVisionTransformer(BaseModule):
"""The backbone of Segformer.
This backbone is the implementation of `SegFormer: Simple and
Efficient Design for Semantic Segmentation with
Transformers <https://arxiv.org/abs/2105.15203>`_.
Args:
in_channels (int): Number of input channels. Default: 3.
embed_dims (int): Embedding dimension. Default: 768.
num_stags (int): The num of stages. Default: 4.
num_layers (Sequence[int]): The layer number of each transformer encode
layer. Default: [3, 4, 6, 3].
num_heads (Sequence[int]): The attention heads of each transformer
encode layer. Default: [1, 2, 4, 8].
patch_sizes (Sequence[int]): The patch_size of each overlapped patch
embedding. Default: [7, 3, 3, 3].
strides (Sequence[int]): The stride of each overlapped patch embedding.
Default: [4, 2, 2, 2].
sr_ratios (Sequence[int]): The spatial reduction rate of each
transformer encode layer. Default: [8, 4, 2, 1].
out_indices (Sequence[int] | int): Output from which stages.
Default: (0, 1, 2, 3).
mlp_ratio (int): ratio of mlp hidden dim to embedding dim.
Default: 4.
qkv_bias (bool): Enable bias for qkv if True. Default: True.
drop_rate (float): Probability of an element to be zeroed.
Default 0.0
attn_drop_rate (float): The drop out rate for attention layer.
Default 0.0
drop_path_rate (float): stochastic depth rate. Default 0.0
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='LN')
act_cfg (dict): The activation config for FFNs.
Default: dict(type='GELU').
pretrained (str, optional): model pretrained path. Default: None.
init_cfg (dict or list[dict], optional): Initialization config dict.
Default: None.
"""
def __init__(self,
in_channels=3,
embed_dims=64,
num_stages=4,
num_layers=[3, 4, 6, 3],
num_heads=[1, 2, 4, 8],
patch_sizes=[7, 3, 3, 3],
strides=[4, 2, 2, 2],
sr_ratios=[8, 4, 2, 1],
out_indices=(0, 1, 2, 3),
mlp_ratio=4,
qkv_bias=True,
drop_rate=0.,
attn_drop_rate=0.,
drop_path_rate=0.,
act_cfg=dict(type='GELU'),
norm_cfg=dict(type='LN', eps=1e-6),
pretrained=None,
init_cfg=None):
super(MixVisionTransformer, self).__init__(init_cfg=init_cfg)
assert not (init_cfg and pretrained), \
'init_cfg and pretrained cannot be set at the same time'
if isinstance(pretrained, str):
warnings.warn('DeprecationWarning: pretrained is deprecated, '
'please use "init_cfg" instead')
self.init_cfg = dict(type='Pretrained', checkpoint=pretrained)
elif pretrained is not None:
raise TypeError('pretrained must be a str or None')
self.embed_dims = embed_dims
self.num_stages = num_stages
self.num_layers = num_layers
self.num_heads = num_heads
self.patch_sizes = patch_sizes
self.strides = strides
self.sr_ratios = sr_ratios
assert num_stages == len(num_layers) == len(num_heads) \
== len(patch_sizes) == len(strides) == len(sr_ratios)
self.out_indices = out_indices
assert max(out_indices) < self.num_stages
# transformer encoder
dpr = [
x.item()
for x in torch.linspace(0, drop_path_rate, sum(num_layers))
] # stochastic num_layer decay rule
cur = 0
self.layers = ModuleList()
for i, num_layer in enumerate(num_layers):
embed_dims_i = embed_dims * num_heads[i]
patch_embed = PatchEmbed(
in_channels=in_channels,
embed_dims=embed_dims_i,
kernel_size=patch_sizes[i],
stride=strides[i],
padding=patch_sizes[i] // 2,
norm_cfg=norm_cfg)
layer = ModuleList([
TransformerEncoderLayer(
embed_dims=embed_dims_i,
num_heads=num_heads[i],
feedforward_channels=mlp_ratio * embed_dims_i,
drop_rate=drop_rate,
attn_drop_rate=attn_drop_rate,
drop_path_rate=dpr[cur + idx],
qkv_bias=qkv_bias,
act_cfg=act_cfg,
norm_cfg=norm_cfg,
sr_ratio=sr_ratios[i]) for idx in range(num_layer)
])
in_channels = embed_dims_i
# The ret[0] of build_norm_layer is norm name.
norm = build_norm_layer(norm_cfg, embed_dims_i)[1]
self.layers.append(ModuleList([patch_embed, layer, norm]))
cur += num_layer
[docs] def init_weights(self):
if self.init_cfg is None:
for m in self.modules():
if isinstance(m, nn.Linear):
trunc_normal_init(m, std=.02, bias=0.)
elif isinstance(m, nn.LayerNorm):
constant_init(m, val=1.0, bias=0.)
elif isinstance(m, nn.Conv2d):
fan_out = m.kernel_size[0] * m.kernel_size[
1] * m.out_channels
fan_out //= m.groups
normal_init(
m, mean=0, std=math.sqrt(2.0 / fan_out), bias=0)
else:
super(MixVisionTransformer, self).init_weights()
[docs] def forward(self, x):
outs = []
for i, layer in enumerate(self.layers):
x, hw_shape = layer[0](x)
for block in layer[1]:
x = block(x, hw_shape)
x = layer[2](x)
x = nlc_to_nchw(x, hw_shape)
if i in self.out_indices:
outs.append(x)
return outs