Source code for mmseg.models.backbones.vit
# Copyright (c) OpenMMLab. All rights reserved.
import math
import warnings
import torch
import torch.nn as nn
from mmcv.cnn import build_norm_layer
from mmcv.cnn.bricks.transformer import FFN, MultiheadAttention
from mmcv.cnn.utils.weight_init import (constant_init, kaiming_init,
trunc_normal_)
from mmcv.runner import (BaseModule, CheckpointLoader, ModuleList,
load_state_dict)
from torch.nn.modules.batchnorm import _BatchNorm
from torch.nn.modules.utils import _pair as to_2tuple
from mmseg.ops import resize
from mmseg.utils import get_root_logger
from ..builder import BACKBONES
from ..utils import PatchEmbed
class TransformerEncoderLayer(BaseModule):
"""Implements one encoder layer in Vision Transformer.
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.
num_fcs (int): The number of fully-connected layers for FFNs.
Default: 2.
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: True.
"""
def __init__(self,
embed_dims,
num_heads,
feedforward_channels,
drop_rate=0.,
attn_drop_rate=0.,
drop_path_rate=0.,
num_fcs=2,
qkv_bias=True,
act_cfg=dict(type='GELU'),
norm_cfg=dict(type='LN'),
batch_first=True):
super(TransformerEncoderLayer, self).__init__()
self.norm1_name, norm1 = build_norm_layer(
norm_cfg, embed_dims, postfix=1)
self.add_module(self.norm1_name, norm1)
self.attn = MultiheadAttention(
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,
bias=qkv_bias)
self.norm2_name, norm2 = build_norm_layer(
norm_cfg, embed_dims, postfix=2)
self.add_module(self.norm2_name, norm2)
self.ffn = FFN(
embed_dims=embed_dims,
feedforward_channels=feedforward_channels,
num_fcs=num_fcs,
ffn_drop=drop_rate,
dropout_layer=dict(type='DropPath', drop_prob=drop_path_rate),
act_cfg=act_cfg)
@property
def norm1(self):
return getattr(self, self.norm1_name)
@property
def norm2(self):
return getattr(self, self.norm2_name)
def forward(self, x):
x = self.attn(self.norm1(x), identity=x)
x = self.ffn(self.norm2(x), identity=x)
return x
[docs]@BACKBONES.register_module()
class VisionTransformer(BaseModule):
"""Vision Transformer.
This backbone is the implementation of `An Image is Worth 16x16 Words:
Transformers for Image Recognition at
Scale <https://arxiv.org/abs/2010.11929>`_.
Args:
img_size (int | tuple): Input image size. Default: 224.
patch_size (int): The patch size. Default: 16.
in_channels (int): Number of input channels. Default: 3.
embed_dims (int): embedding dimension. Default: 768.
num_layers (int): depth of transformer. Default: 12.
num_heads (int): number of attention heads. Default: 12.
mlp_ratio (int): ratio of mlp hidden dim to embedding dim.
Default: 4.
out_indices (list | tuple | int): Output from which stages.
Default: -1.
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
with_cls_token (bool): Whether concatenating class token into image
tokens as transformer input. Default: True.
output_cls_token (bool): Whether output the cls_token. If set True,
`with_cls_token` must be True. Default: False.
norm_cfg (dict): Config dict for normalization layer.
Default: dict(type='LN')
act_cfg (dict): The activation config for FFNs.
Default: dict(type='GELU').
patch_norm (bool): Whether to add a norm in PatchEmbed Block.
Default: False.
final_norm (bool): Whether to add a additional layer to normalize
final feature map. Default: False.
interpolate_mode (str): Select the interpolate mode for position
embeding vector resize. Default: bicubic.
num_fcs (int): The number of fully-connected layers for FFNs.
Default: 2.
norm_eval (bool): Whether to set norm layers to eval mode, namely,
freeze running stats (mean and var). Note: Effect on Batch Norm
and its variants only. Default: False.
with_cp (bool): Use checkpoint or not. Using checkpoint will save
some memory while slowing down the training speed. Default: False.
pretrained (str, optional): model pretrained path. Default: None.
init_cfg (dict or list[dict], optional): Initialization config dict.
Default: None.
"""
def __init__(self,
img_size=224,
patch_size=16,
in_channels=3,
embed_dims=768,
num_layers=12,
num_heads=12,
mlp_ratio=4,
out_indices=-1,
qkv_bias=True,
drop_rate=0.,
attn_drop_rate=0.,
drop_path_rate=0.,
with_cls_token=True,
output_cls_token=False,
norm_cfg=dict(type='LN'),
act_cfg=dict(type='GELU'),
patch_norm=False,
final_norm=False,
interpolate_mode='bicubic',
num_fcs=2,
norm_eval=False,
with_cp=False,
pretrained=None,
init_cfg=None):
super(VisionTransformer, self).__init__(init_cfg=init_cfg)
if isinstance(img_size, int):
img_size = to_2tuple(img_size)
elif isinstance(img_size, tuple):
if len(img_size) == 1:
img_size = to_2tuple(img_size[0])
assert len(img_size) == 2, \
f'The size of image should have length 1 or 2, ' \
f'but got {len(img_size)}'
if output_cls_token:
assert with_cls_token is True, f'with_cls_token must be True if' \
f'set output_cls_token to True, but got {with_cls_token}'
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.img_size = img_size
self.patch_size = patch_size
self.interpolate_mode = interpolate_mode
self.norm_eval = norm_eval
self.with_cp = with_cp
self.pretrained = pretrained
self.patch_embed = PatchEmbed(
in_channels=in_channels,
embed_dims=embed_dims,
conv_type='Conv2d',
kernel_size=patch_size,
stride=patch_size,
padding='corner',
norm_cfg=norm_cfg if patch_norm else None,
init_cfg=None,
)
num_patches = (img_size[0] // patch_size) * \
(img_size[1] // patch_size)
self.with_cls_token = with_cls_token
self.output_cls_token = output_cls_token
self.cls_token = nn.Parameter(torch.zeros(1, 1, embed_dims))
self.pos_embed = nn.Parameter(
torch.zeros(1, num_patches + 1, embed_dims))
self.drop_after_pos = nn.Dropout(p=drop_rate)
if isinstance(out_indices, int):
if out_indices == -1:
out_indices = num_layers - 1
self.out_indices = [out_indices]
elif isinstance(out_indices, list) or isinstance(out_indices, tuple):
self.out_indices = out_indices
else:
raise TypeError('out_indices must be type of int, list or tuple')
dpr = [
x.item() for x in torch.linspace(0, drop_path_rate, num_layers)
] # stochastic depth decay rule
self.layers = ModuleList()
for i in range(num_layers):
self.layers.append(
TransformerEncoderLayer(
embed_dims=embed_dims,
num_heads=num_heads,
feedforward_channels=mlp_ratio * embed_dims,
attn_drop_rate=attn_drop_rate,
drop_rate=drop_rate,
drop_path_rate=dpr[i],
num_fcs=num_fcs,
qkv_bias=qkv_bias,
act_cfg=act_cfg,
norm_cfg=norm_cfg,
batch_first=True))
self.final_norm = final_norm
if final_norm:
self.norm1_name, norm1 = build_norm_layer(
norm_cfg, embed_dims, postfix=1)
self.add_module(self.norm1_name, norm1)
@property
def norm1(self):
return getattr(self, self.norm1_name)
[docs] def init_weights(self):
if (isinstance(self.init_cfg, dict)
and self.init_cfg.get('type') == 'Pretrained'):
logger = get_root_logger()
checkpoint = CheckpointLoader.load_checkpoint(
self.init_cfg['checkpoint'], logger=logger, map_location='cpu')
if 'state_dict' in checkpoint:
state_dict = checkpoint['state_dict']
else:
state_dict = checkpoint
if 'pos_embed' in state_dict.keys():
if self.pos_embed.shape != state_dict['pos_embed'].shape:
logger.info(msg=f'Resize the pos_embed shape from '
f'{state_dict["pos_embed"].shape} to '
f'{self.pos_embed.shape}')
h, w = self.img_size
pos_size = int(
math.sqrt(state_dict['pos_embed'].shape[1] - 1))
state_dict['pos_embed'] = self.resize_pos_embed(
state_dict['pos_embed'],
(h // self.patch_size, w // self.patch_size),
(pos_size, pos_size), self.interpolate_mode)
load_state_dict(self, state_dict, strict=False, logger=logger)
elif self.init_cfg is not None:
super(VisionTransformer, self).init_weights()
else:
# We only implement the 'jax_impl' initialization implemented at
# https://github.com/rwightman/pytorch-image-models/blob/master/timm/models/vision_transformer.py#L353 # noqa: E501
trunc_normal_(self.pos_embed, std=.02)
trunc_normal_(self.cls_token, std=.02)
for n, m in self.named_modules():
if isinstance(m, nn.Linear):
trunc_normal_(m.weight, std=.02)
if m.bias is not None:
if 'ffn' in n:
nn.init.normal_(m.bias, mean=0., std=1e-6)
else:
nn.init.constant_(m.bias, 0)
elif isinstance(m, nn.Conv2d):
kaiming_init(m, mode='fan_in', bias=0.)
elif isinstance(m, (_BatchNorm, nn.GroupNorm, nn.LayerNorm)):
constant_init(m, val=1.0, bias=0.)
def _pos_embeding(self, patched_img, hw_shape, pos_embed):
"""Positiong embeding method.
Resize the pos_embed, if the input image size doesn't match
the training size.
Args:
patched_img (torch.Tensor): The patched image, it should be
shape of [B, L1, C].
hw_shape (tuple): The downsampled image resolution.
pos_embed (torch.Tensor): The pos_embed weighs, it should be
shape of [B, L2, c].
Return:
torch.Tensor: The pos encoded image feature.
"""
assert patched_img.ndim == 3 and pos_embed.ndim == 3, \
'the shapes of patched_img and pos_embed must be [B, L, C]'
x_len, pos_len = patched_img.shape[1], pos_embed.shape[1]
if x_len != pos_len:
if pos_len == (self.img_size[0] // self.patch_size) * (
self.img_size[1] // self.patch_size) + 1:
pos_h = self.img_size[0] // self.patch_size
pos_w = self.img_size[1] // self.patch_size
else:
raise ValueError(
'Unexpected shape of pos_embed, got {}.'.format(
pos_embed.shape))
pos_embed = self.resize_pos_embed(pos_embed, hw_shape,
(pos_h, pos_w),
self.interpolate_mode)
return self.drop_after_pos(patched_img + pos_embed)
[docs] @staticmethod
def resize_pos_embed(pos_embed, input_shpae, pos_shape, mode):
"""Resize pos_embed weights.
Resize pos_embed using bicubic interpolate method.
Args:
pos_embed (torch.Tensor): Position embedding weights.
input_shpae (tuple): Tuple for (downsampled input image height,
downsampled input image width).
pos_shape (tuple): The resolution of downsampled origin training
image.
mode (str): Algorithm used for upsampling:
``'nearest'`` | ``'linear'`` | ``'bilinear'`` | ``'bicubic'`` |
``'trilinear'``. Default: ``'nearest'``
Return:
torch.Tensor: The resized pos_embed of shape [B, L_new, C]
"""
assert pos_embed.ndim == 3, 'shape of pos_embed must be [B, L, C]'
pos_h, pos_w = pos_shape
cls_token_weight = pos_embed[:, 0]
pos_embed_weight = pos_embed[:, (-1 * pos_h * pos_w):]
pos_embed_weight = pos_embed_weight.reshape(
1, pos_h, pos_w, pos_embed.shape[2]).permute(0, 3, 1, 2)
pos_embed_weight = resize(
pos_embed_weight, size=input_shpae, align_corners=False, mode=mode)
cls_token_weight = cls_token_weight.unsqueeze(1)
pos_embed_weight = torch.flatten(pos_embed_weight, 2).transpose(1, 2)
pos_embed = torch.cat((cls_token_weight, pos_embed_weight), dim=1)
return pos_embed
[docs] def forward(self, inputs):
B = inputs.shape[0]
x, hw_shape = self.patch_embed(inputs)
# stole cls_tokens impl from Phil Wang, thanks
cls_tokens = self.cls_token.expand(B, -1, -1)
x = torch.cat((cls_tokens, x), dim=1)
x = self._pos_embeding(x, hw_shape, self.pos_embed)
if not self.with_cls_token:
# Remove class token for transformer encoder input
x = x[:, 1:]
outs = []
for i, layer in enumerate(self.layers):
x = layer(x)
if i == len(self.layers) - 1:
if self.final_norm:
x = self.norm1(x)
if i in self.out_indices:
if self.with_cls_token:
# Remove class token and reshape token for decoder head
out = x[:, 1:]
else:
out = x
B, _, C = out.shape
out = out.reshape(B, hw_shape[0], hw_shape[1],
C).permute(0, 3, 1, 2).contiguous()
if self.output_cls_token:
out = [out, x[:, 0]]
outs.append(out)
return tuple(outs)
[docs] def train(self, mode=True):
super(VisionTransformer, self).train(mode)
if mode and self.norm_eval:
for m in self.modules():
if isinstance(m, nn.LayerNorm):
m.eval()