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# Ultralytics YOLO 🚀, AGPL-3.0 license
"""Transformer modules."""
import math
import torch
import torch.nn as nn
import torch.nn.functional as F
from torch.nn.init import constant_, xavier_uniform_
from .conv import Conv
from .utils import _get_clones, inverse_sigmoid, multi_scale_deformable_attn_pytorch
__all__ = (
"TransformerEncoderLayer",
"TransformerLayer",
"TransformerBlock",
"MLPBlock",
"LayerNorm2d",
"AIFI",
"DeformableTransformerDecoder",
"DeformableTransformerDecoderLayer",
"MSDeformAttn",
"MLP",
)
class TransformerEncoderLayer(nn.Module):
"""Defines a single layer of the transformer encoder."""
def __init__(self, c1, cm=2048, num_heads=8, dropout=0.0, act=nn.GELU(), normalize_before=False):
"""Initialize the TransformerEncoderLayer with specified parameters."""
super().__init__()
from ...utils.torch_utils import TORCH_1_9
if not TORCH_1_9:
raise ModuleNotFoundError(
"TransformerEncoderLayer() requires torch>=1.9 to use nn.MultiheadAttention(batch_first=True)."
)
self.ma = nn.MultiheadAttention(c1, num_heads, dropout=dropout, batch_first=True)
# Implementation of Feedforward model
self.fc1 = nn.Linear(c1, cm)
self.fc2 = nn.Linear(cm, c1)
self.norm1 = nn.LayerNorm(c1)
self.norm2 = nn.LayerNorm(c1)
self.dropout = nn.Dropout(dropout)
self.dropout1 = nn.Dropout(dropout)
self.dropout2 = nn.Dropout(dropout)
self.act = act
self.normalize_before = normalize_before
@staticmethod
def with_pos_embed(tensor, pos=None):
"""Add position embeddings to the tensor if provided."""
return tensor if pos is None else tensor + pos
def forward_post(self, src, src_mask=None, src_key_padding_mask=None, pos=None):
"""Performs forward pass with post-normalization."""
q = k = self.with_pos_embed(src, pos)
src2 = self.ma(q, k, value=src, attn_mask=src_mask, key_padding_mask=src_key_padding_mask)[0]
src = src + self.dropout1(src2)
src = self.norm1(src)
src2 = self.fc2(self.dropout(self.act(self.fc1(src))))
src = src + self.dropout2(src2)
return self.norm2(src)
def forward_pre(self, src, src_mask=None, src_key_padding_mask=None, pos=None):
"""Performs forward pass with pre-normalization."""
src2 = self.norm1(src)
q = k = self.with_pos_embed(src2, pos)
src2 = self.ma(q, k, value=src2, attn_mask=src_mask, key_padding_mask=src_key_padding_mask)[0]
src = src + self.dropout1(src2)
src2 = self.norm2(src)
src2 = self.fc2(self.dropout(self.act(self.fc1(src2))))
return src + self.dropout2(src2)
def forward(self, src, src_mask=None, src_key_padding_mask=None, pos=None):
"""Forward propagates the input through the encoder module."""
if self.normalize_before:
return self.forward_pre(src, src_mask, src_key_padding_mask, pos)
return self.forward_post(src, src_mask, src_key_padding_mask, pos)
class AIFI(TransformerEncoderLayer):
"""Defines the AIFI transformer layer."""
def __init__(self, c1, cm=2048, num_heads=8, dropout=0, act=nn.GELU(), normalize_before=False):
"""Initialize the AIFI instance with specified parameters."""
super().__init__(c1, cm, num_heads, dropout, act, normalize_before)
def forward(self, x):
"""Forward pass for the AIFI transformer layer."""
c, h, w = x.shape[1:]
pos_embed = self.build_2d_sincos_position_embedding(w, h, c)
# Flatten [B, C, H, W] to [B, HxW, C]
x = super().forward(x.flatten(2).permute(0, 2, 1), pos=pos_embed.to(device=x.device, dtype=x.dtype))
return x.permute(0, 2, 1).view([-1, c, h, w]).contiguous()
@staticmethod
def build_2d_sincos_position_embedding(w, h, embed_dim=256, temperature=10000.0):
"""Builds 2D sine-cosine position embedding."""
assert embed_dim % 4 == 0, "Embed dimension must be divisible by 4 for 2D sin-cos position embedding"
grid_w = torch.arange(w, dtype=torch.float32)
grid_h = torch.arange(h, dtype=torch.float32)
grid_w, grid_h = torch.meshgrid(grid_w, grid_h, indexing="ij")
pos_dim = embed_dim // 4
omega = torch.arange(pos_dim, dtype=torch.float32) / pos_dim
omega = 1.0 / (temperature**omega)
out_w = grid_w.flatten()[..., None] @ omega[None]
out_h = grid_h.flatten()[..., None] @ omega[None]
return torch.cat([torch.sin(out_w), torch.cos(out_w), torch.sin(out_h), torch.cos(out_h)], 1)[None]
class TransformerLayer(nn.Module):
"""Transformer layer https://arxiv.org/abs/2010.11929 (LayerNorm layers removed for better performance)."""
def __init__(self, c, num_heads):
"""Initializes a self-attention mechanism using linear transformations and multi-head attention."""
super().__init__()
self.q = nn.Linear(c, c, bias=False)
self.k = nn.Linear(c, c, bias=False)
self.v = nn.Linear(c, c, bias=False)
self.ma = nn.MultiheadAttention(embed_dim=c, num_heads=num_heads)
self.fc1 = nn.Linear(c, c, bias=False)
self.fc2 = nn.Linear(c, c, bias=False)
def forward(self, x):
"""Apply a transformer block to the input x and return the output."""
x = self.ma(self.q(x), self.k(x), self.v(x))[0] + x
return self.fc2(self.fc1(x)) + x
class TransformerBlock(nn.Module):
"""Vision Transformer https://arxiv.org/abs/2010.11929."""
def __init__(self, c1, c2, num_heads, num_layers):
"""Initialize a Transformer module with position embedding and specified number of heads and layers."""
super().__init__()
self.conv = None
if c1 != c2:
self.conv = Conv(c1, c2)
self.linear = nn.Linear(c2, c2) # learnable position embedding
self.tr = nn.Sequential(*(TransformerLayer(c2, num_heads) for _ in range(num_layers)))
self.c2 = c2
def forward(self, x):
"""Forward propagates the input through the bottleneck module."""
if self.conv is not None:
x = self.conv(x)
b, _, w, h = x.shape
p = x.flatten(2).permute(2, 0, 1)
return self.tr(p + self.linear(p)).permute(1, 2, 0).reshape(b, self.c2, w, h)
class MLPBlock(nn.Module):
"""Implements a single block of a multi-layer perceptron."""
def __init__(self, embedding_dim, mlp_dim, act=nn.GELU):
"""Initialize the MLPBlock with specified embedding dimension, MLP dimension, and activation function."""
super().__init__()
self.lin1 = nn.Linear(embedding_dim, mlp_dim)
self.lin2 = nn.Linear(mlp_dim, embedding_dim)
self.act = act()
def forward(self, x: torch.Tensor) -> torch.Tensor:
"""Forward pass for the MLPBlock."""
return self.lin2(self.act(self.lin1(x)))
class MLP(nn.Module):
"""Implements a simple multi-layer perceptron (also called FFN)."""
def __init__(self, input_dim, hidden_dim, output_dim, num_layers, act=nn.ReLU, sigmoid=False):
"""Initialize the MLP with specified input, hidden, output dimensions and number of layers."""
super().__init__()
self.num_layers = num_layers
h = [hidden_dim] * (num_layers - 1)
self.layers = nn.ModuleList(nn.Linear(n, k) for n, k in zip([input_dim] + h, h + [output_dim]))
self.sigmoid = sigmoid
self.act = act()
def forward(self, x):
"""Forward pass for the entire MLP."""
for i, layer in enumerate(self.layers):
x = getattr(self, "act", nn.ReLU())(layer(x)) if i < self.num_layers - 1 else layer(x)
return x.sigmoid() if getattr(self, "sigmoid", False) else x
class LayerNorm2d(nn.Module):
"""
2D Layer Normalization module inspired by Detectron2 and ConvNeXt implementations.
Original implementations in
https://github.com/facebookresearch/detectron2/blob/main/detectron2/layers/batch_norm.py
and
https://github.com/facebookresearch/ConvNeXt/blob/main/models/convnext.py.
"""
def __init__(self, num_channels, eps=1e-6):
"""Initialize LayerNorm2d with the given parameters."""
super().__init__()
self.weight = nn.Parameter(torch.ones(num_channels))
self.bias = nn.Parameter(torch.zeros(num_channels))
self.eps = eps
def forward(self, x):
"""Perform forward pass for 2D layer normalization."""
u = x.mean(1, keepdim=True)
s = (x - u).pow(2).mean(1, keepdim=True)
x = (x - u) / torch.sqrt(s + self.eps)
return self.weight[:, None, None] * x + self.bias[:, None, None]
class MSDeformAttn(nn.Module):
"""
Multiscale Deformable Attention Module based on Deformable-DETR and PaddleDetection implementations.
https://github.com/fundamentalvision/Deformable-DETR/blob/main/models/ops/modules/ms_deform_attn.py
"""
def __init__(self, d_model=256, n_levels=4, n_heads=8, n_points=4):
"""Initialize MSDeformAttn with the given parameters."""
super().__init__()
if d_model % n_heads != 0:
raise ValueError(f"d_model must be divisible by n_heads, but got {d_model} and {n_heads}")
_d_per_head = d_model // n_heads
# Better to set _d_per_head to a power of 2 which is more efficient in a CUDA implementation
assert _d_per_head * n_heads == d_model, "`d_model` must be divisible by `n_heads`"
self.im2col_step = 64
self.d_model = d_model
self.n_levels = n_levels
self.n_heads = n_heads
self.n_points = n_points
self.sampling_offsets = nn.Linear(d_model, n_heads * n_levels * n_points * 2)
self.attention_weights = nn.Linear(d_model, n_heads * n_levels * n_points)
self.value_proj = nn.Linear(d_model, d_model)
self.output_proj = nn.Linear(d_model, d_model)
self._reset_parameters()
def _reset_parameters(self):
"""Reset module parameters."""
constant_(self.sampling_offsets.weight.data, 0.0)
thetas = torch.arange(self.n_heads, dtype=torch.float32) * (2.0 * math.pi / self.n_heads)
grid_init = torch.stack([thetas.cos(), thetas.sin()], -1)
grid_init = (
(grid_init / grid_init.abs().max(-1, keepdim=True)[0])
.view(self.n_heads, 1, 1, 2)
.repeat(1, self.n_levels, self.n_points, 1)
)
for i in range(self.n_points):
grid_init[:, :, i, :] *= i + 1
with torch.no_grad():
self.sampling_offsets.bias = nn.Parameter(grid_init.view(-1))
constant_(self.attention_weights.weight.data, 0.0)
constant_(self.attention_weights.bias.data, 0.0)
xavier_uniform_(self.value_proj.weight.data)
constant_(self.value_proj.bias.data, 0.0)
xavier_uniform_(self.output_proj.weight.data)
constant_(self.output_proj.bias.data, 0.0)
def forward(self, query, refer_bbox, value, value_shapes, value_mask=None):
"""
Perform forward pass for multiscale deformable attention.
https://github.com/PaddlePaddle/PaddleDetection/blob/develop/ppdet/modeling/transformers/deformable_transformer.py
Args:
query (torch.Tensor): [bs, query_length, C]
refer_bbox (torch.Tensor): [bs, query_length, n_levels, 2], range in [0, 1], top-left (0,0),
bottom-right (1, 1), including padding area
value (torch.Tensor): [bs, value_length, C]
value_shapes (List): [n_levels, 2], [(H_0, W_0), (H_1, W_1), ..., (H_{L-1}, W_{L-1})]
value_mask (Tensor): [bs, value_length], True for non-padding elements, False for padding elements
Returns:
output (Tensor): [bs, Length_{query}, C]
"""
bs, len_q = query.shape[:2]
len_v = value.shape[1]
assert sum(s[0] * s[1] for s in value_shapes) == len_v
value = self.value_proj(value)
if value_mask is not None:
value = value.masked_fill(value_mask[..., None], float(0))
value = value.view(bs, len_v, self.n_heads, self.d_model // self.n_heads)
sampling_offsets = self.sampling_offsets(query).view(bs, len_q, self.n_heads, self.n_levels, self.n_points, 2)
attention_weights = self.attention_weights(query).view(bs, len_q, self.n_heads, self.n_levels * self.n_points)
attention_weights = F.softmax(attention_weights, -1).view(bs, len_q, self.n_heads, self.n_levels, self.n_points)
# N, Len_q, n_heads, n_levels, n_points, 2
num_points = refer_bbox.shape[-1]
if num_points == 2:
offset_normalizer = torch.as_tensor(value_shapes, dtype=query.dtype, device=query.device).flip(-1)
add = sampling_offsets / offset_normalizer[None, None, None, :, None, :]
sampling_locations = refer_bbox[:, :, None, :, None, :] + add
elif num_points == 4:
add = sampling_offsets / self.n_points * refer_bbox[:, :, None, :, None, 2:] * 0.5
sampling_locations = refer_bbox[:, :, None, :, None, :2] + add
else:
raise ValueError(f"Last dim of reference_points must be 2 or 4, but got {num_points}.")
output = multi_scale_deformable_attn_pytorch(value, value_shapes, sampling_locations, attention_weights)
return self.output_proj(output)
class DeformableTransformerDecoderLayer(nn.Module):
"""
Deformable Transformer Decoder Layer inspired by PaddleDetection and Deformable-DETR implementations.
https://github.com/PaddlePaddle/PaddleDetection/blob/develop/ppdet/modeling/transformers/deformable_transformer.py
https://github.com/fundamentalvision/Deformable-DETR/blob/main/models/deformable_transformer.py
"""
def __init__(self, d_model=256, n_heads=8, d_ffn=1024, dropout=0.0, act=nn.ReLU(), n_levels=4, n_points=4):
"""Initialize the DeformableTransformerDecoderLayer with the given parameters."""
super().__init__()
# Self attention
self.self_attn = nn.MultiheadAttention(d_model, n_heads, dropout=dropout)
self.dropout1 = nn.Dropout(dropout)
self.norm1 = nn.LayerNorm(d_model)
# Cross attention
self.cross_attn = MSDeformAttn(d_model, n_levels, n_heads, n_points)
self.dropout2 = nn.Dropout(dropout)
self.norm2 = nn.LayerNorm(d_model)
# FFN
self.linear1 = nn.Linear(d_model, d_ffn)
self.act = act
self.dropout3 = nn.Dropout(dropout)
self.linear2 = nn.Linear(d_ffn, d_model)
self.dropout4 = nn.Dropout(dropout)
self.norm3 = nn.LayerNorm(d_model)
@staticmethod
def with_pos_embed(tensor, pos):
"""Add positional embeddings to the input tensor, if provided."""
return tensor if pos is None else tensor + pos
def forward_ffn(self, tgt):
"""Perform forward pass through the Feed-Forward Network part of the layer."""
tgt2 = self.linear2(self.dropout3(self.act(self.linear1(tgt))))
tgt = tgt + self.dropout4(tgt2)
return self.norm3(tgt)
def forward(self, embed, refer_bbox, feats, shapes, padding_mask=None, attn_mask=None, query_pos=None):
"""Perform the forward pass through the entire decoder layer."""
# Self attention
q = k = self.with_pos_embed(embed, query_pos)
tgt = self.self_attn(q.transpose(0, 1), k.transpose(0, 1), embed.transpose(0, 1), attn_mask=attn_mask)[
0
].transpose(0, 1)
embed = embed + self.dropout1(tgt)
embed = self.norm1(embed)
# Cross attention
tgt = self.cross_attn(
self.with_pos_embed(embed, query_pos), refer_bbox.unsqueeze(2), feats, shapes, padding_mask
)
embed = embed + self.dropout2(tgt)
embed = self.norm2(embed)
# FFN
return self.forward_ffn(embed)
class DeformableTransformerDecoder(nn.Module):
"""
Implementation of Deformable Transformer Decoder based on PaddleDetection.
https://github.com/PaddlePaddle/PaddleDetection/blob/develop/ppdet/modeling/transformers/deformable_transformer.py
"""
def __init__(self, hidden_dim, decoder_layer, num_layers, eval_idx=-1):
"""Initialize the DeformableTransformerDecoder with the given parameters."""
super().__init__()
self.layers = _get_clones(decoder_layer, num_layers)
self.num_layers = num_layers
self.hidden_dim = hidden_dim
self.eval_idx = eval_idx if eval_idx >= 0 else num_layers + eval_idx
def forward(
self,
embed, # decoder embeddings
refer_bbox, # anchor
feats, # image features
shapes, # feature shapes
bbox_head,
score_head,
pos_mlp,
attn_mask=None,
padding_mask=None,
):
"""Perform the forward pass through the entire decoder."""
output = embed
dec_bboxes = []
dec_cls = []
last_refined_bbox = None
refer_bbox = refer_bbox.sigmoid()
for i, layer in enumerate(self.layers):
output = layer(output, refer_bbox, feats, shapes, padding_mask, attn_mask, pos_mlp(refer_bbox))
bbox = bbox_head[i](output)
refined_bbox = torch.sigmoid(bbox + inverse_sigmoid(refer_bbox))
if self.training:
dec_cls.append(score_head[i](output))
if i == 0:
dec_bboxes.append(refined_bbox)
else:
dec_bboxes.append(torch.sigmoid(bbox + inverse_sigmoid(last_refined_bbox)))
elif i == self.eval_idx:
dec_cls.append(score_head[i](output))
dec_bboxes.append(refined_bbox)
break
last_refined_bbox = refined_bbox
refer_bbox = refined_bbox.detach() if self.training else refined_bbox
return torch.stack(dec_bboxes), torch.stack(dec_cls)
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