ViTAE: Vision Transformer Advanced by Exploring Intrinsic Inductive Bias
Authors: Yufei Xu
Qiming ZHANG
Jing Zhang
Dacheng Tao
Venue: NeurIPS 2021 Poster (2021)
Keywords: Vision Transformer
Classification
Convolution
Inductive Bias

Abstract: Transformers have shown great potential in various computer vision tasks owing to their strong capability in modeling long-range dependency using the self-attention mechanism. Nevertheless, vision transformers treat an image as 1D sequence of visual tokens, lacking an intrinsic inductive bias (IB) in modeling local visual structures and dealing with scale variance. Alternatively, they require large-scale training data and longer training schedules to learn the IB implicitly. In this paper, we propose a new Vision Transformer Advanced by Exploring intrinsic IB from convolutions, i.e., ViTAE. Technically, ViTAE has several spatial pyramid reduction modules to downsample and embed the input image into tokens with rich multi-scale context by using multiple convolutions with different dilation rates. In this way, it acquires an intrinsic scale invariance IB and is able to learn robust feature representation for objects at various scales. Moreover, in each transformer layer, ViTAE has a convolution block in parallel to the multi-head self-attention module, whose features are fused and fed into the feed-forward network. Consequently, it has the intrinsic locality IB and is able to learn local features and global dependencies collaboratively. Experiments on ImageNet as well as downstream tasks prove the superiority of ViTAE over the baseline transformer and concurrent works. Source code and pretrained models will be available at https://github.com/Annbless/ViTAE.

ViTAE: Vision Transformer Advanced by Exploring Intrinsic Inductive Bias
Yufei Xu
Qiming Zhang
Jing Zhang'
Dacheng Tao
'Te University of Sydney, Australia, 2J Explore Academy, China
fyuxu7116, 22m.ii00..2a2200mmm..m... exydee.y .wydney. edu.au, au, jing. chaang@@yddeey.co eewpdddeyyedu...eem .du.au, au, dacheng. tao0gmail.... com
Abstract
Transformers have shown great potential various computer vision tasks owing to their strong capability in modeling long-range dependency using the self-attention mechanism. Nevertheless, vision transformers treat an image as 1D sequence of visual tokens, lacking intrinsic inductive bias (IB) in modeling local visual structures and dealing with scale variance. Alternatively. they require large-scale training data and longer training schedules learn the IB implicitly. this paper, we propose a new Vision Transformer Advanced by Exploring intrinsic IB from convolutions, i.e., VITAE. Technically, ViTAE has several spatial pyramid reduction modules to downsample and embed the input image into tokens with rich multi-scale context by using multiple convolutions with different dilation rates. In this way, it acquires an intrinsic scale invariance IB and is able to learn robust feature representation for objects at various scales. Moreover, in each transformer layer, VITAE has convolution block in parallel to the multi-head self- attention module, whose features are fused and fed into the feed-forward network. Consequently, it has the intrinsic locality IB and is able to learn local features and global dependencies collaboratively. Experiments on ImageNet as well as downstream tasks prove the superiority of VITAE over the baseline transformer and concurrent works. Source code and pretrained models will be available at code
Introduction
73
T2T
75.3
72.6
have shown a domination trend in NLP studies owing to their strong ability in modeling long-range dependen-
Transformers 779 17 40
14
46
61
VITAE
Imogrhe Top-1 Actrayy (%)
71.6
71.2
71
cies by the self-attention mechanism 667 81 [51.
72.6
69
Such success and good properties of transform- ers has inspired following many works that apply
60,7
68.1
71.7
67.5
Intepeat
them in various computer vision tasks [19 100
70.8
97 80 [7]. Among them, ViT [19] is the pionecr- ing pure transformer model that embeds images into a sequence of visual tokens and models the global dependencies among them with stacked transformer blocks. Although achieves promis- ing performance image classification, quires large-scale training data and a longer train- ing schedule. One important reason is that ViT
65
61.1
68.7
63
20
60
100
68
100
200
300
Percentage
Epochs
Figure 1: Comparison of data and training effi- ciency of T2T-ViT-7 and ViTAE-T on ImageNet.
Equal Contribution. Interns JD Explore Academy.
35th Conference on Neural Information Processing Systems (NcurIPS 2021).

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lacks intrinsic inductive bias (IB) in modeling local visual structures (e.g.. edges and corners) and dealing with objects various scales like convolutions. Alternatively, ViT has to learn such IB implicitly from large-scale data.
Unlike vision transformers, Convolution Neural Networks (CNNs) naturally equip with the intrinsic IBs of scale-invariance and locality and still serve as prevalent backbones in vision tasks [26, 770 62 8 96). The success of CNNs inspires us to explore intrinsic IBs in vision transformers. We start by analyzing the above two IBs of CNNs, i.e., locality and scale-invariance. Convolution that computes local correlation among neighbor pixels is good at extracting local features such as edges and corners. Consequently, CNNs can provide plentiful low-level features at the shallow layers [94], which are then aggregated into high-level features progressively by a bulk of sequential convolutions (7]. Moreover, CNNs have a bierarchy structure to extract multi-scale features at different 32 68
layers 688 38 26] Besides, intra-layer convolutions can also learn features at different scales by 70
45 96]. Consequently, scale- invariant feature representation can be obtained via intra- or inter-layer feature fusion. Nevertheless, CNNs are not well suited to model long-range dependencies ? which is the key advantage of transformers. An interesting question comes up: Can we improve vision transformers by leveraging the good properties of CNNs? Recently, DeiT (76) explores the idea of distilling knowledge from CNNs to transformers to facilitate training and improve the performance. However, it requires an off-the-shelf CNN model as the teacher and consumes extra training cost.
varying their kernel sizes and dilation rates (25
8
Different from DeiT, we explicitly introduce intrinsic IBs into vision transformers by re-designigg the network structures in this paper. Current vision transformers always obtain tokens with single- 77 and learn to adapt to objects at different scales from data. improves VIT by delicately generating tokens in a soft split manner.
scale context [19 93 80 86
69
For example, VIT
93
Specifically, it uses a series of Tokens-to- Token transformation layers to aggregate single-scale neighboring contextual information and progressively structurizes the image to tokens. Motivated by the success of CNNs in dealing with scale variance, we explore a similar design in transformers, i.., intra-layer convolutions with different receptive fields [70 [1), to embed multi-scale context into tokens. Such a design allows tokens to carry useful features of objects at various scales, thereby naturally having the intrinsic scale- invariance IB and explicitly facilitating transformers to learn scale-invariant features more efficiently from data. On the other hand, low-level local features are fundamental elements to generate high-level discriminative features. Although transformers can also learn such features at shallow layers from data, they are not skilled as convolutions by design. stack convolutions and attention layers sequentially and demonstrate that locality is a reasonable compensation of global dependency. However, this serial structure ignores the global context during locality modeling (and vice versa). To avoid such a dilemma, we follow the ddivide-and-conques idea and propose to model locality and long-range dependencies in parallel and then fuse the features to account for both. In this way, we empower transformers to learn local and long-range features within each block more effectively.
Recently, [8 43 21
Technically, propose new Vision Transformers Advanced by Exploring Intrinsic Inductive Bias (ViTAE), which is a combination of two types of basic cells, Le., reduction cell (RC) and normal cell (NC). RCs are used downsample and embed the input images into tokens with rich multi-scale context while NCs aim to jointly model locality and global dependencies in the token sequence. Moreover, these two types of cells share simple basic structure, i.e., paralleled attention module and convolutional layers followed by a feed-forward network (FFN). It is noteworthy that RC has an extra pyramid reduction module with atrous convolutions of different dilation rates embed multi-scale context into tokens. Following the setting in (93. we stack three reduction cells to reduce the spatial resolution by 1/16 and a series of NCs to learn discriminative features from data. ViTAE outperforms representative vision transformers in terms of data efficiency and training efficiency (see Figure []. as well as classification accuracy and generalization on downstream tasks.
Our contributions are threefold. First, we explore two types of intrinsic IB in transformers, i.e., scale invariance and locality, and demonstrate the effectiveness of this idea in improving the feature learning ability of transformers. Second, we design new transformer architecture named VITAE based two new reduction and normal cells intrinsically incorporate the above two IBs. The proposed ViTAE embeds multi-scale context into tokens and learns both local and long-range features
'Dspptt projection in transformer can be viewed as 1 x 1 convolution 9 the term of convolution here refers to those with larger kernels, e.g., 3 x 3, which are widely used in typical CNNs to extract spatial features.

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effectively. Third, ViTAE outperforms representative vision transformers regarding classification accuracy, data efficiency, training efficiency, and generalization on downstream tasks. ViTAE achieves 75.3% and 82.0% top-1 accuracy on ImageNet with 4.8M and 23.6M parameters, respectively.
2 Related Work
2.1 CNNS with intrinsic IB
87 and downstream computer vision tasks. The convolution operations in CNNs extract local features from the neighbor pixels within the receptive field determined by the kernel size 42 Following the intuition that local pixels are more likely to be correlated in images [41), CNNs have the intrinsic IB in modeling locality. In addition the locality, another critical topic in visual tasks is scale-invariance, where multi-scale features are needed to represent the objects at different scales effectively [49] (90]. For example, to effectively learn features of large objects, large receptive field is needed by either using large convolution kernels 990 91 or series of convolution layers in deeper architectures [26] 32
CNNs have led to series of breakthroughs in image classification 38 94 26 95
68
77. To construct multi-scale feature representation, the classical idea is using image pyramid 39 16]. where features are
or learned from pyramid of images at different Accordingly, features from the small scale image mainly encode the large objects while features from the large scale image respond more to small objects. addition to the above inter-layer fusion way, another way to aggregate multi-scale context by using multiple convolutions with different receptive fields within single layer, i.e., intra-layer fusion 196 71 70 70 72). Either inter-layer fusion or intra-layer fusion empower CNNs an intrinsic IB in modeling scale-invariance. This paper introduces such an IB to vision transformers by following the intra-layer fusion idea and utilizing multiple convolutions with different dilation rates in the reduction cells to encode multi-scale context into each visual token.
8 55
resolutions respectively 44 52
63
35
3
2.2 Vision transformers with learned IB
VIT [9] is the pioneering work that applies a pure transformer to vision tasks and achieves promising results. However, since VIT lacks intrinsic inductive bias in modeling local visual structures, it indeed
learns the IB from amounts of data implicitly. Following works along this direction are to simplify
the model structures with fewer intrinsic IBs and directly learn them from large scale data [50 74 75
22 18 20 27 which have achieved promising results and been studied actively. Another direction is
to leverage the intrinsic IB from CNNs facilitate the training of vision transformers, e.g., using less training data or shorter training schedules. For example, DeiT (76) proposes to distill knowledge from CNNs transformers during training. However, it requires an off-the-shelf CNN model teacher, introducing extra computation cost during training. Recently, some works try to introduce the intrinsic For example,
IB of CNNs into vision transformers explicitly (23 58 21
43
15
89
83
92
stack convolutions and attention layers sequentially, resulting in a serial structure and modeling the locality and global dependency accordingly. 80
43
21 3
28 design sequential stage-wise structures while 44 333 apply attention within local windows. However, these serial structure may ignore the global context during locality modeling (and vice versa). [88] establishes connection across different scales at the cost of heavy computation. Instead, we follow the "divide-and "awiid--nd-conquer" idea and propose to model locality and global dependencies simultaneously via a parallel structure within each transformer layer. Conformer (58), the most relevant concurrent work to us, employs unit to explore inter-block interactions between parallel convolution and transformer blocks. In contrast, in ViTAE, the convolution and attention modules are designed to be complementary each other within the transformer block. In addition, Conformer is not designed to have inherent scale invariance IB.
3 Methodology
3.1 Revisit vision transformer
We first give brief review of vision transformer in this part. To adapt transformers to vision lasks, VIT 19 first splits an image x E xW xC' into tokens with a reduction ratio of p (i.e.,
Ie (((Hxx)))')))) where H. W and C' denote the height, width, and channel dimensions of
3

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B.l. M.8
8706 W710
DO0-10
000-1 00
000
Reductionn Call
Redictionn
-000-00
Norial Call
Samman Cormeaa Call
Namall Call
Liwaxr
Dog
-0-0
00000
SILL
INN
Pyramid Reduccton
Lowe
Difherent Dilstion Rates
3 Position Embodding
Element-wise dd
Feature Map
Concat
Token
Class Token
The structure of the proposed VITAE. is constructed by stacking three RCs and several NCs. Both types of cells share a simple basic structure. i.e., an MHSA module and parallel convolutional module followed by an FFN. particular, RC has an extra pyramid reduction module using atrous convolutions with different dilation rates to embed multi-scale context into tokens.
Figure 2:
the input image, = Cp² denotes the token dimension. Then, an extra class token concatenated to the visual tokens before adding position embeddings in element-wise manner. The resulting tokens are fed into the following transformer layers. Each transformer layer is composed of two parts, i.., multi-head self-attention module (MHSA) and feed forward network (FFN).
MHSA Multi-head self-attention extends single-head self-attention (SHSA) by using different projection matrices for each head. Specifically, the input tokens H; are first projected to queries (Q), keys (K) and values (V) using projection matrices, i.e., Q, K, V I, We x.QK IvQv where Wo/k/V RDx denotes the projection matrix for query, key, and value, respectively. Then, the self-attention operation is calculated as:
QKT VD )V.
Attention (acettion(,HAee (Q, KVV) = softmart
(1)
This SHSA module is repeated for h times to formulate the MHSA module, where h is the number of heads. The output features the h heads are concatenated along the channel dimension and formulate the output of the MHSA module.
FFN FFN is placed on top of the MHSA module and applied each token identically and separately. consists of two linear transformations with an activation function in between. Besides, a layer normalization 2 and shortcut are added before and aside from the MHSA and FFN, respectively.
3.2 Overview architecture of VITAE
ViTAE aims to introduce the intrinsic IB in CNNs to vision transformers. As shown in Figure 2 ViTAE composed of two types of cells, i.e., RCs and NCs. RCs are responsible for embedding multi-scale context and local information into tokens, and NCs are used to further model the locality and long-range dependencies the tokens. Taken image
# xC', as input, three RCs are used to gradually downsample by 4x, 2x, and 2x, respectively. Thereby, the output tokens of the token dimension (64 in our experiments). The
the RCs are of size H /1, W/16, D) where output tokens RCs are then flattened as RHW/256XD concatenated with the class token, and added by the sinusoid position encoding. Next, the tokens are fed into the following NCs, which keep the length of the tokens. Finally, the prediction probability obtained using a linear classification layer on the class token from the last NC.

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3.3 Reduction cell
Instead of directly splitting and flatten images into visual tokens based on a linear image patch embedding layer, we devise the reduction cell to embed multi-scale context and local information into visual tokens, which introduces the intrinsic scale-invariance and locality IBs from convolutions. Technically, RC has two parallel branches responsible for modeling locality and long-range depen- dency, respectively, followed by an FFN for feature transformation. We denote the input feature of the ieh RC as fi RBixWExDD D. The input of the first RC is the image æ. In the global dependencies branch, fi is firstly fed into a Pyramid Reduction Module (PRM) to extract multi-scale context, i.e.,
fTT & PRM((F) (a sig, rig) E Si, E RJ),
(2)
where Convey) (-) indicates the jth convolutional layer in the PRM (PRM, It uses a dilation rate Sty from the predefined dilation rate set S; corresponding to the ith RC. Note that we use stride convolution to reduce the spatial dimension of features by a ratio r; from the predefined reduction ratio set R. The conv features are concatenated along the channel dimension, i.e., f! e (( (B..s))) ((S.)) where |8:] denotes the number of dilation rates in S. f is then processed by an MHSA module to model long-range dependencies, i.e.
f = MWS.AMAAAAmm(((((.
where /mg2Seg(-) is a simple reshape operation to flatten the feature map to ID sequence. In this way, f embeds the multi-scale context each token. In addition, we use a Parallel Convolutional Module (PCM) to embed local context within the tokens, which are fused with f as follows:
(4)
Here, PCM((-) represents the PCM, which is composed of three stacked convolution layers and an (mg2Seqq-) operation. It is noteworthy that the parallel convolution branch has the same spatial downsampling ratio the PRM by using stride convolutions. In this way, the token features can carry both local and multi-scale context, implying that RC acquires the locality IB and scale-invariance IB by design. The fused tokens are then processed by the FFN, reshaped back to feature maps, and fed into the following RC or NC, i.e.,
f++ SeQ2Img(FNN + f.").
(5)
where the Seq2Img(-) is a simple reshape operation to reshape a token sequence back to feature maps. (-N,(-) represents the FFN in the ith RC. In our VITAE, three RCs are stacked sequentially to gradually reduce the input image's spatial dimension by 4x, 2x, and 2x, respectively. The feature maps generated by the last RC are of size of [H/16, W/16, D], which are then flattened into visual tokens and fed into the following NCs.
3.4 Normal cell
As shown in the bottom right part of Figure 2 except for the absence of the PRM. Due the relatively small 1 x) spatial size of feature maps after RCs, is unnecessary to use PRM in NCs. Given fa from the third RC, we first concatenate it with the class token fole, and then add it to the positional encodings to get the input tokens for the following NCs. Here we ignore the subscript for clarity since all NCs have an identical architecture but different learnable weights. fets is randomly initialized at the start of training and fixed during the inference. Similar to the RC, the tokens are fed into the MHSA module, i.e., ty = H.S.A A(t). Meanwhile, they Img2Seq
NCs share a similar structure with the reduction cell
reshaped to 2D feature maps and fed into the PCM, i.e., Note that the class token is discarded in PCM because it has no spatial connections with other visual tokens. To further reduce the parameters in NCs, from MHSA and PCM are then fused via element-wise sum, Le., hig = by + 4.- Finally, 4g are fed into the FFN to get the output features of NC, i.e., tnc FFN (tay) + tig. Similar VIT [19), we apply layer normalization to the class token generated by the last NC and feed it to the classification head get the final classification result.
group convolutions in PCM. The features
3.5 Model details
5

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We use two variants of VITAE in our experiments for fair com- parison of other models with sim- ilar model sizes. The details of them are summarized in [ In the first RC, the default convo- lution kernel size is 7 x 7 with stride of 4 and dilation rates of S [1, 2, 4]. In the following two RCs, the convolution kernel
Table Model details of two variants of ViTAE.
Reduction Cell
Normal Cell
Params Maes
Model VITAE-T
Dilation
Cells
Heads Embed Cells
(M)
(G)
1,2,3,4
3
4
256
4.8
1.5
VITAE-S
1,2,3,4
3
6
384
14
23.6
5.6
size is 3 x with stride of 2 and dilation rates of S₂ [1, and Sy [1, 2], respectively. Since the spatial dimension of tokens decreases, there is no need to use large kernels and dilation rates.
PCM in both RCs and NCs comprises three convolutional layers with kernel size of 3 x 3.
4 Experiments
4.1
Implementation details
We train and test the proposed ViTAE model on the standard ImageNet [38] dataset, which contains about 1.3 million images and covers IK classes. Unless explicitly stated, the image size during training is set to 224 x 224. We use the Adam W [48] optimizer with the cosine learning rate scheduler and uses the data augmentation strategy exactly the same as T2T [93] for fair comparison, regarding the training strategies and the size of models. We use a batch size of 512 for training all our models and
set the initial learning rate be 5e-4. The results of our models can be found in Table 2 where all
the models are trained for 300 epochs on 8 V100 GPUs. The models are built on Py PyTorch [57 and TIMM 82)
4.2 Comparison with the state-of-the-art
We compare our VITAE with both CNN models and vision transformers with similar model sizes in Table 2 Both Top-1/5 accuracy and real Top-1 accuracy on the ImageNet validation set are reported. We categorize the methods into CNN models, vision transformers with learned IB, and vision transformers with introduced intrinsic IB. Compared with CNN models, our ViTAE-T achieves 75.3% Top-1 accuracy, which is better than ResNet-18 18 with more parameters. The real Top-1 accuracy of the VITAE model is 82.9%, which is comparable to ResNet-50 that has four more times ViTAE-S achieves 82.0% Top-1 accuracy with half of the
of parameters than ours. Similarly, parameters of ResNet-101 101 and ResNet-152 showing the superiority of learning both local and long- range features from specific structures with corresponding intrinsic IBs by design. Similar phenomena can also be observed when comparing ViTAE-T with MobileNetV1 [1] and MobileNetV2 [65], where VITAE obtains better performance with fewer parameters. When compared with larger models which are searched according to NAS 773, our ViTAE-S achieves a similar performance when using 384 x 384 images input, which further shows the potential of vision transformers with intrinsic IB.
In addition, among the transformers with learned IB, ViT is the first pure transformer model for visual recognition. DeiT shares the same structure with VIT but uses different data augmentation and training strategies to facilitate the learning of transformers. Adenotts using an off-the- off-the-shelf CNN model the teacher model to train DeiT, which introduces the intrinsic IB from CNN transformer implicitly in knowledge distillation manner, showing better performance than the vanilla ViT on the ImageNet dataset. It is exciting to see that our ViTAE-T with fewer parameters even outperforms the distilled model De demonstrating the efficacy of introducing intrinsic IBs in transformers by design. Besides, compared with other transformers with explicit intrinsic IB, our ViTAE with fewer parameters also achieves comparable or better performance. For instance, VITAE-T achieves comparable performance with LocalVit-T but has IM fewer parameters, demonstrating the superiority of the proposed RCs and NCs in introducing intrinsic IBs.
4.3 Ablation study
We use T2T-ViT 933 as our baseline model in the following ablation study of our VITAE, As shown in Table 3, we investigate the hyper-parameter settings in RCs and NCs by isolating them separately. All the models are trained for 100 epochs on ImageNet and follow the same training setting and data augmentation strategy as described in Section

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Table 2: Comparison of VITAR and SOTA methods on the ImageNet validation set.
Params
MACs
Input Size
ImageNet
Real
Type
Model
(M)
(G)
Top-1
Top-5
Top-1
ResNet-18 26
11.7
3.6
224
70.3
86.7
77.3
ResNet-50
26
25.6
7.6
224
76.7
93.3
82.5
ResNet-101
26
44.5
15.2
224
78.3
94.1
83.7
ResNet-152 26
60.2
22.6
224
78.9
94.4
84.1
EfficientNNtt
76
5.3
0.8
224
77.1
93.3
83.5
CNN
EfficiemmNNc:-B444
19.3
8.4
380
82.9
96.4
88.0
MobileNetV1
4.3
224
72.3
MobileNatV2((.44) 655
6.9
0.6
224
74.7
RegNetY-600M 62
6.1
1.2
224
75.5
RegNetY-44FF 62
20.6
8.0
224
80.0
86.4
RegNetY-8GF
39,2
16.0
224
81.7
DeiT-T 76
5.7
2.6
224
72,2
91.1
80.6
DeiT-TM 76
5.7
2.6
224
74.5
91.9
82.1
LocalViT-T 4k
5.9
2.6
224
74.8
92.6
LocaIViT-T2T
4.3
2.4
224
72,5
ConT-Ti 89
5.8
1.6
224
74.9
PiT-Ti 22
4.9
1.4
224
73.0
T2T-ViT-7 93
4.3
1.2
224
71.7
90.9
79.7
VITAE-T
4.8
1.5
224
75.3
92.7
82.9
VITAE-T 384
4.8
5.7
384
77.2
93.8
84.4
CeiT-T 922
6.4
224
76.4
93.4
83.6
ConViT-Ti
15
6.0
2.0
224
73.1
CrossViT-Ti
6
6.9
3.2
224
73.4
VITAE-6M
6.5
2.0
224
77.9
94.1
84.9
PVT-T 80
13.2
3.8
224
75.1
LocalViT-PVT
43
13.5
9.6
224
78.2
94.2
ConViT-Ti+
10.0
4.0
224
76.7
PÝT-XS
10.6
2.8
224
78.1
ConTIM 89
19.2
6.2
224
80.2
ViTAE-13M
13.2
3.4
224
81.0
95.4
86.8
DeiT-S 776
22.1
9.8
224
79.9
95.0
85.7
DeiT-S 76
22.1
9.8
224
81.2
95.4
86.8
PVT-S 80
24.5
7.6
79.8
Conformer-1
58
23.5
5.2
224
81.3
Swin-T 47
29.0
9.0
224
81.3
CeiT-S 92
24.2
9.0
224
82.0
95.9
87.3
Transformer
CvT-13
20.0
224
81.6
86.7
ConViT-S
15
27.0
10.8
224
81.3
Cio-s
26.7
11.2
224
81.0
PYT-S 20
23.5
4.8
224
80.9
TNT-S 23
23.8
10.4
224
81.3
95.6
Twins-PCPVT-S/IO
24.1
7.4
224
81.2
Twins-SVT-S 100
24.0
5.6
224
81.7
T2T-ViT-14 98]
21.5
5.2
224
81.5
95.7
86.8
VITAE-S
23.6
5.6
224
82.0
95.9
87.0
VITAE-S 384
23.6
20.2
384
83.0
96.2
87.5

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We use v and X to denote whether or not the corresponding module is enabled during the experiments. If all columns under the RC and NC are marked x shown in the first row, the model becomes the standard TiT-ViT model. "Pre" indicates the output features of PCM and MHSA are fused before FFN while "Post" indicates late fusion strategy correspondingly. "BN" indicates whether PCM uses BN after the convolutional layer or not. "x3" in the first column denotes that the dilation rate set is the same in the three RCs, "[1, 2, 4] 1" denotes using lower dilation rates in deeper RCs, i.e.,
S [1, 2, 3, 4]. S2 [1, 2, 3). S3 [1.
As can be seen, using a pre-fusion strategy and BN achieves the best 69.9% Top-1 accuracy among other settings. It is noteworthy that all the variants of NC outperform the vanilla T2T-ViT, implying the effectiveness of PCM, which introduces the intrin- sic locality IB in transformers. It can also be observed that BN plays an im- portant role in improving the model's performance as it can help to allevi- ate the scale deviation between convo- lution's 's and attention's 's features. For the RC, we first investigate the impact of using different dilation rates in the PRM, shown in the first column. As can be seen, using larger dilation rates (e.g., 4 or 5) does not deliver better performance. We suspect that larger dilation rates may lead to plain features in the deeper RCs due to the smaller resolution of feature maps. To validate the hypothesis, we use smaller dilation rates in deeper RCs as denoted by 2,, 3, 4. As
Table 3: Ablation Study of RCs and NCs in our VITAE. "Pre" indicates the output features of PCM and MHSA are fused before FFN while "Post" indicates late fusion strategy correspondingly. "BN" indicates whether PCM uses BN or not. "[1, 2, 3, 4] 1"' denotes using smaller dilation rates in deeper RCs, i.e., S1 = [1, 2,3, 3, 4), S2 = 21 S3 [1 2].
Reduction Cell
Normal Cell
Top-1
Dilation (Si ~ Sa))
PCM
Pre
Post
BN
X
68.7
x
X
V
X
x
69.1
X
x
X
V
x
69.0
X
V
68.8
V
69.9
[1,2] x 3
X
X
X
69.5
[1,2.3
x
X
X
X
X
69.9
1,
4 x
69.2
[1,
11,2,3,4,
x3
68,9
1,
X
X
69.8
1,
X
X
71.7
1,2,3,4
72.6
can be seen, achieves comparable performance [1, 2, 3] x. However, compared with [1, 3, 4] L. [1, 2, 3] x increases the amount of parameters from 4.35M to 4.6M. Therefore, we select [1. 2, 3, 4| as the default setting. In addition, after using PCM in the RC, it introduces the intrinsic locality IB, and the performance increases to 71.7% Top-1 accuracy. Finally, the combination of RCs and NCs achieves the best accuracy at 72.6%, demonstrating the complementarity between our RCs and NCs.
4.4 Data efficiency and training efficiency
To validate the effectiveness of the introduced intrinsic IBs in improving data efficiency and training efficiency, we compare our ViTAE with T2T-ViT at different training settings: (a) training them using 20%, 60%, and 100% ImageNet training set for equivalent 100 epochs on the full ImageNet training set, e.g., we employ times epochs when using 20% data for training compared with using 100% data; and (b) training them using the full ImageNet training set for 100, 200, and 300 epochs respectively. The results are shown in Figure [ As can be seen, VYTAE consistently outperforms the T2T-ViT baseline by large margin in terms of both data efficiency and training efficiency. For example, VITAE using only 20% training data achieves comparable performance with T2T-ViT using all data. When 60% training data are used, ViTAE significantly outperforms T2T-ViT using all data by about an absolute 3% accuracy. It is also noteworthy that ViTAE trained for only 100 epochs has outperformed T2T-ViT trained for 300 epochs. After training VITAE for 300 epochs, its performance is significantly boosted to 75.3% Top-1 accuracy. With the proposed RCs and NCs, the transformer layers in our VÎTAE only need to focus on modeling rangrangge dependencies, leaving the locality and multi-scale context modeling to its convolution counterparts, ie.. PCM and PRM. Such "divide-amd aeriide-nd--oon and--andddooe conquer"" strategy facilitates the training of vision transformers, making it possible to learn more efficiently with less training data and fewer training epochs.
To further validate the data efficiency of ViTAE model, we train the VITAE model from scratch on the smaller datasets, i.e., Cifar and C0far100. The results are summarized in Table It can be viewed that with only 1/7 number of epochs, the ViTAE-T model achieves better classification
8

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performance on Cifar10 dataset, with far fewer parameters (4.8M v.s. 86M), which further confirms ViTAE model's data efficiency.
Table 4: Results of training from scratch on Cifar
Model
Params (M)
Top-1 Acc
Epochs
Dataset
DeiT-B
86.0
97.5
7000
Cifar10
V
4.8
97.7
1000
Cifar10
VITAE-T
4.8
85.0
1000
Cifar100
4.5
Generalization on downstream tasks
Table 5: Generalization of ViTAE and SOTA methods different downstream tasks.
Model
Params (M)
Cifarlo
Cifar100
iNat19
Cars
Flowers
Pets
Grafit ResNet-50 [78
25.6
75.9
92.5
98.2
EfficientNet-B3 BB 77
30
98.1
91.1
98.5
ViT-B/16 19
86.5
98.1
87.1
89.5
93.8
VIT-L/16 19
304.3
97.9
86.4
89.7
93.6
DeïT-B 76
86.6
99.1
77.7
92.1
98.4
T2T-ViT-14 993
21.5
98.3
88.4
VITAE-T
4.8
97.3
86.0
73.3
89.5
97.5
92.6
VITAE-S
23.6
98.8
90.8
76.0
91.4
97.8
94.2
We further investigate the generalization of the proposed VITAE models on downstream tasks by fine- tuning them on the training sets of several fine-graiie fineggrained classification tasks including Flowers [33
Cars [36]. Pets [56] and INtturalist99, We also fine-tune the proposed ViTAE models Cifar10 37
and Cifar100 37 The results are shown in Table
can be seen that ViTAE achieves SOTA
performance on most of the datasets using comparable or fewer parameters. These results demonstrate that the good generalization ability of our VITAE.
4.6 Visual inspection VITAE
To further analyze the property of our VITAE, we first calculate the average attention distance of each layer in ViTAE-T and the baseline T2T-ViT-7 on the ImageNet test set, respectively. The results are shown in Figure 3 It can be observed that with the usage of PCM. which focuses modeling locality, the transformer layers in the proposed NCs can better focus on modeling long-range dependencies, especially in shallow layers. In the deep layers, the average attention distances of ViTAE-T and T2T-ViT-7 are almost the same since modeling long-range dependencies much more important. These results confirm the effectiveness of the adopted "divide-and-conquee" idea in the proposed ViTAE, i.e., introducing the intrinsic locality IB from con- volutions into vision transformers makes it possible that transformer layers only need to be responsible long-range dependencies, since locality can be well modeled by convolutions in PCM.
T2F-VIT
VITAE
Attention Distance (pkoal)
100
80
6
Besides, we apply Grad-CAM (66) on the MHSA's output in the last NC qualitatively inspect ViTAE. The visualization results are provided Figure Compared with the baseline T2T-VIT, our VITAE covers the single or multiple targets in the images more precisely and attends less to the background. Moreover, VITAE better handle the scale variance issue as shown Figure (b)). Namely, it can precisely cover the birds no matter they are in small, middle, large size. Such observations demonstrate that introducing the intrinsic IBs of locality and scale-invariance from convolutions to transformers helps ViTAE learn more discriminate features than the pure transformers.
Figure 3: The average per-layer attention dis- tance of T2T VTT-7 and our ViTAE-T.
'The performance of VITAE on dense prediction tasks such as detection, segmentation, pose estimation, can be found in the supplementary material.

---

Imput
T2I-VIT
VITAE
(8)
(b)
Figure 4: Visual inspection of T2T-ViT and ViTAE using Grad-CAM [66]. (a) Images containing multiple or single objects and the heatmaps. (b) Images containing the same class of objects at different scales and the heatmaps (Best viewed in color).
5 Limitation and discussion
In this paper, we explore two types of IBs and incorporate them into transformers through the proposed reduction and normal cells. With the collaboration these two cells, our ViTAE model achieves impressive performance on the ImageNet with fast convergence and high data efficiency, Nevertheless, due to computational resource constraints, we not scaled the ViTAE model and train it on large- size dataset, e.g., ImageNet-2IK (38) and JFT-300M [30]- Although remains unclear by now, are optimistic about its scale property from the following preliminary evidence. As illustrated in Figure our ViTAE model can be viewed an intra-cell ensemble of complementary transformer layers and convolution layers owing to the skip connection and parallel structure. According to the attention distance analysis shown Figure 3 the ensemble nature enables the transformer layers and convolution layers to focus on what they are good at, i.e., modeling long-rang dependencies and locality. Therefore, VITAE is very likely learn better feature representation from large- acage data. Besides, we only study two typical IBs in this paper. More kinds of IBs such as constituting viewpoint invariance [64] can be explored in the future study.
Conclusion
In this paper, we re-design the transformer block by proposing two basic cells (reduction cells and normal cells) incorporate two types intrinsic inductive bias (IB) into transformers, i.e., locality and scale-invariance, resulting in simple yet effective vision transformer architecture named VITAE. Extensive experiments show that ViTAE outperforms representative vision transformers in various respects including classification accuracy, data efficiency, training efficiency, and generalization ability on downstream tasks. We plan to scale ViTAE to the large or huge model size and train on large-size datasets in the future study. In addition, other kinds of IBs will also be investigated. We hope that this study will provide valuable insights to the following studies introducing intrinsic into vision transformers and understanding the impact of intrinsic and learned IBs.
Acknowledgement Dr. Jing Zhang is supported by the ARC project FL- 17010700
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14


Reviews:
[{"review_id": "lFoZHCzqMXO", "reviewer": "Reviewer_seSX", "summary": "The paper propose to inject inductive bias (IB) for vision tasks to boost the performance of vision transformers (ViT). The main contributions of the paper are two-fold: a reduction cell that aggregate the features from different scales by controlling the dilation of a  few of parallel convolution branches and a parallel convolution branch with the multi-head self-attention (MHSA) block. The results on ImageNet is promising compared with the selected baselines but I found it is hard to get a conclusive result on downstream classification tasks.\n", "questions": "", "limitations": "", "rating": 6, "confidence": 4, "soundness": null, "presentation": null, "contribution": null, "strengths": "", "weaknesses": "", "quality": null, "clarity": null, "significance": null, "originality": null, "strengths_and_weaknesses": "", "was_revised": false, "final_justification": "", "rebuttal": "", "claims_and_evidence": "", "methods_and_evaluation_criteria": "", "theoretical_claims": "", "experimental_designs_or_analyses": "", "relation_to_broader_scientific_literature": "", "essential_references_not_discussed": "", "other_strengths_and_weaknesses": "", "other_comments_or_suggestions": "", "questions_for_authors": "", "summary_of_paper": "", "summary_of_recommendation": "", "technical_quality": "", "clarity_of_presentation": "", "potential_impact": "", "robotics_focus": "", "quality_of_limitations": "", "questions_for_rebuttal": "", "reasons_to_accept": "", "reasons_to_reject": "", "questions_to_authors": "", "main_review": "Strong aspects:\n1. The idea of injecting multi-scale features is interesting and promising.\n2. The paper is well written and easy to follow.\n\nWeakness:\n\n1. Novelty: The main contribution of the paper is the multi-scale token aggregation of the image tokens. This is interesting as the token downsampling has been used by some recent papers and shown to be beneficial to tradeoff between the model accuracy and the resource consumption. Introduce a multi-scale reduction cell is reasonable and could protentional mitigate the information loss. However, as the experiments in the paper are mainly based on the single object classification datasets, it is not clear how many benefits come from the hand-crafted multi-scale feature aggregation. Results on multi-scale datasets could be more convincing.\n\n2. Ablation experiments. As the contribution of the paper is two fold, it would be clearer to have an ablation of the impacts of each component. For an example, what if only reduction cell is used with following block the same as the original MHSA blocks?  \n\n3. As the original MHSA block structure also learns the global relationship, the multi-scale features could have already been learned. Why using hand-crafted multi-scale convolution branch could improve the final accuracy?\n\n4. Concerns on the model latency and memory consumption. In the reduction blocks and normal blocks, multi-branch convolutions are used. The proposed method is mainly verified on small input images. Thus, I am a little bit concerned about its memory consumption and running speed when applied on large images (as segmentation or detection typically uses large image resolutions).\n\nI would be happy to increase my score if the authors could address my concerns sufficiently.\n", "summary_of_the_paper": "", "summary_of_the_review": "", "strength_and_weaknesses": "", "limitations_and_societal_impact": "Yes", "review_text": "", "extra_scores": {"needs_ethics_review": "No", "ethics_review_area": "", "time_spent_reviewing": "8"}, "extra_text": {}}, {"review_id": "88YdnDtK2ij", "reviewer": "Reviewer_kqV3", "summary": "This paper proposed two types of basic cells to modify the vanilla vision transformer structure: reduction cell (RC) and normal cell (NC). RCs are used to down-sample and embed the input images into tokens and NCs are to jointly model locality and global dependencies in the token sequence. Specifically, they used paralleled attention module and convolutional layers followed by a feed-forward network (FFN) to construct these two cells. Experiments on ImageNet and downstream tasks verified the superiority of the proposed network design.", "questions": "", "limitations": "", "rating": 5, "confidence": 5, "soundness": null, "presentation": null, "contribution": null, "strengths": "", "weaknesses": "", "quality": null, "clarity": null, "significance": null, "originality": null, "strengths_and_weaknesses": "", "was_revised": false, "final_justification": "", "rebuttal": "", "claims_and_evidence": "", "methods_and_evaluation_criteria": "", "theoretical_claims": "", "experimental_designs_or_analyses": "", "relation_to_broader_scientific_literature": "", "essential_references_not_discussed": "", "other_strengths_and_weaknesses": "", "other_comments_or_suggestions": "", "questions_for_authors": "", "summary_of_paper": "", "summary_of_recommendation": "", "technical_quality": "", "clarity_of_presentation": "", "potential_impact": "", "robotics_focus": "", "quality_of_limitations": "", "questions_for_rebuttal": "", "reasons_to_accept": "", "reasons_to_reject": "", "questions_to_authors": "", "main_review": "Strengths:\n\n1. The paper is well-presented.\n\n2. Consider the intrinsic inductive bias in modeling local visual structures and scale variance in transformer for vision modality is reasonable.\n\nWeaknesses:\n\n1. This paper proposed to use an additional conv branch together with the self-attention branch to construct the new network architecture, it is obvious that the extra conv layers will help to improve the performance of the network. However, more conv layers will definitely involve more computational cost. My main concern of this paper is that the proposed network modification looks a little bit incremental and not very interesting to me.\n\n2. For the experimental part, the performance is also not competitive if comparing to many recently proposed vision transformer works. I understand it may be too fastidious to consider these newly released papers, but I think using the additional branch of conv layers combining with the self-attention module, the improvement is expected, meanwhile, more FLOPs will be involved.\n\n3. There are no results on the downstream object detection and segmentation tasks, since this paper aims to introduce the inductive bias on the visual structure. These downstream tasks are more useful in real-world applications so it will be better to see whether the proposed structure modification can consistently help on these tasks.\n", "summary_of_the_paper": "", "summary_of_the_review": "", "strength_and_weaknesses": "", "limitations_and_societal_impact": "The network modification with more convolutional layers in this paper looks a little bit incremental and less interesting. I feel the contribution is not significant but my final rating will consider the other reviewers’ comments.", "review_text": "", "extra_scores": {"needs_ethics_review": "No", "ethics_review_area": "", "time_spent_reviewing": "4 hours"}, "extra_text": {}}, {"review_id": "0v5PRsEzAO2", "reviewer": "Reviewer_dhAk", "summary": "This work introduces a kind of convolution path in the transformer architecture. The main goal is to incorporate in the vision transformer architecture the intrinsic inductive bias (for instance, for modeling local visual structures) from the convolution. ", "questions": "", "limitations": "", "rating": 5, "confidence": 5, "soundness": null, "presentation": null, "contribution": null, "strengths": "", "weaknesses": "", "quality": null, "clarity": null, "significance": null, "originality": null, "strengths_and_weaknesses": "", "was_revised": false, "final_justification": "", "rebuttal": "", "claims_and_evidence": "", "methods_and_evaluation_criteria": "", "theoretical_claims": "", "experimental_designs_or_analyses": "", "relation_to_broader_scientific_literature": "", "essential_references_not_discussed": "", "other_strengths_and_weaknesses": "", "other_comments_or_suggestions": "", "questions_for_authors": "", "summary_of_paper": "", "summary_of_recommendation": "", "technical_quality": "", "clarity_of_presentation": "", "potential_impact": "", "robotics_focus": "", "quality_of_limitations": "", "questions_for_rebuttal": "", "reasons_to_accept": "", "reasons_to_reject": "", "questions_to_authors": "", "main_review": "\n1. The approach is limited in novelty as this path of introducing convolutions in transformers was already explored in the previous works (for instance, in Conformer [46] and LocalViT [32]).\n\n2. The results are unconvincing. In Table 2 the work claims superior performance over the other approaches but there are some question marks. For instance, the authors report for T2T-ViT-14 only the results with 224x224 crop size but they did not report also the results with 384x384 crop size, where T2T-ViT-14↑384 obtain an accuracy of 83.3% (see Table 2 in T2T work https://arxiv.org/pdf/2101.11986.pdf). This is curious because T2T-ViT-14↑384 outperforms the proposed approach (which obtains for ViTAE-S↑384 an accuracy of 83.0%), importantly T2T-ViT-14↑384 requires lower number of parameters and FLOPS than ViTAE-S↑384. This is especially concerning as the proposed approach is built on top of T2T, thus in this comparison the proposed approach shows even worse results than its baseline. Furthermore, this work did not report the results of T2T with both cases: transformer and performer in T2T module (see, for instance, Table 3 in T2T work https://arxiv.org/pdf/2101.11986.pdf). This may show an unfair/misleading comparison. \n\n3. At line 246 the authors highlighted: “It is exciting to see that our ViTAE-T with fewer parameters even outperforms the distilled model DeiT⚗, demonstrating the efficacy of introducing intrinsic IBs in transformers by design.” This is a quite unfair direct comparison, as the proposed approach is built on top of T2T, thus it accumulates the benefits from T2T and DeiT did not benefit from T2T improvement. \n\n4. Unclear results. In Table 3, the pre- and post-fusion have quite similar results 69.1% vs. 69%, thus there is no clear choice. However, when adding the BN, the pre-fusion outperforms significantly the post-fusion, which looks quite questionable. What is the reason that BN influences so critically the fusion type?\n\n5. It is important to have reported also in Table 3 the number of parameters and FLOPs for each configuration. It is not very clear that these multiple dilations really help. What are the results of using just a convolution with a single dilation of 1? What are the results of using PCM and not using multiple dilations (just using the convolution with a single dilation of 1)?\n\n6. Besides ImageNet, the work does not train from scratch on any other dataset (just performs the fine-tuning on other datasets). As the authors claim data efficiency and training efficiency, it is important to see how the approach performs when training from scratch on other datasets. Particularly, it can be important to see how it performs on smaller datasets (for instance, CIFAR10/100) where in general the vision transformer architectures did not perform that well.  ", "summary_of_the_paper": "", "summary_of_the_review": "", "strength_and_weaknesses": "", "limitations_and_societal_impact": "-", "review_text": "", "extra_scores": {"ethical_concerns": "-", "needs_ethics_review": "No", "ethics_review_area": "", "time_spent_reviewing": "4"}, "extra_text": {}}]

Author rebuttal:

Meta-review: This paper augments vision transformers by using spatial pyramid downsamples to create multiple scales for learning, thereby introducing scale invariance as an inductive bias. Using this approach, the authors show improved performance on ImageNet and several downstream tasks across a number of metrics, including training and data efficiency. Reviewers rated the paper as borderline, with concerns regarding the novelty with respect to other hybrid model approaches and proper evaluation of baselines. The authors provided a substantial response to the reviewer concerns, but unfortunately, several of the reviewers did not respond to the author rebuttal. In my reading, the authors were able to address a number of concerns, but some points remain unclear, particularly with respect to comparison to prior models. As such, this paper would likely benefit from further work before publication.
Decision: Accept (Poster)