# Review — Learning Deep Transformer Models for Machine Translation

## Pre-Norm Transformer, Layer Normalization First

Learning Deep Transformer Models for Machine TranslationPre-Norm Transformer, by Northeastern University, NiuTrans Co., Ltd., Kingsoft AI Lab, and University of Macau2019 ACL, Over 200 Citations(Sik-Ho Tsang @ Medium)

Natural Language Processing, NLP, Machine Translation, Transformer

- Two novel techniques are proposed:

- A proper use of layer normalization is proposed, called
**pre-norm****Transformer**, and; - A novel way of
**passing the combination of previous layers to the next**.

# Outline

**Pre-Norm****Transformer****Dynamic Linear Combination of Layers (DLCL)****Experimental Results**

**1. Pre-Norm **Transformer

## 1.1. **Original Post-Norm ****Transformer**

- On the encoder side, there are a number of identical stacked layers. Each of them is composed of a self-attention sub-layer and a feed-forward sub-layer.
- The attention model used in Transformer is multi-head attention, and its output is fed into a fully connected feed-forward network.
- For Transformer, it is
**not easy to train stacked layers**,**residual connections**and**layer normalization**are adopted:

- where
*xl*and*xl*+1 are the input and output of the*l*-th sub-layer, and*yl*is the intermediate output followed by the post-processing function*f*(). **Layer normalization****is placed after the element-wise residual addition**:

- It can be seen as a
**post-processing step**of the output.

## 1.2. **Proposed Pre-Norm **Transformer

- In contrast, layer normalization is
**applied to the input of every sub-layer**:

- The above equation regards layer normalization
**as a part of the sub-layer**, and does nothing for post-processing of the residual connection.

## 1.3. Gradients of Post-Norm

**A stack of**are used as an example. Let*L*sub-layersbe the*E***loss**used to measure how many errors occur in system prediction, andbe the*xL***output of the topmost sub-layer**.- For
**post-norm****Transformer**, given a sub-layer*l*,**the differential of**can be computed by the chain rule, and we have:*E*with respect to xl

The above equation is

inefficient for passing gradients backbecause the residual connection isnot a bypassof the layer normalization unit.

## 1.4. Gradients of Pre-Norm

- Likewise, we have the
**gradient for pre-norm**:

Obviously, the above equation for

pre-norm establishes a direct way to pass error gradient ∂. Its merit lies in that the number of product items on the right sideE/∂xLfrom top to bottomdoes not depend on the depth of the stack.

# 2. Dynamic Linear Combination of Layers (DLCL)

DLCL is proposed to make

direct links with all previous layersandoffers efficient access to lower-level representations in a deep stack.

- Let
**{**be the*y*0, …,*yl*}**output of layers 0~**. The input of layer*l**l*+1 is defined to be:

- where
*G*() is a linear function that merges previously generated values {*y*0, …,*yl*} into a new value. - For
**pre-norm****Transformer**,is defined as:*G*()

where

is aW(l+1)klearnable scalarandweights each incoming layer in a linear manner. The above equation provides a way tolearn preference of layers in different levels of the stack.

- For
**post-norm**,can be redefined as:*G*()

# 3. Experimental Results

## 3.1. SOTA Comparison

- When increasing the encoder depth, e.g.
*L*=20, the vanilla**Transformer****failed to train.**On the contrary,**post-norm DLCL solves this issue**and achieves the**best result when***L*=25.

Pre-norm slightly underperforms the post-norm counterpart in shallow networks, pre-norm Transformerbenefits more from the increase in encoder depth.

**Pre-norm is easier to optimize**than post-norm in deep networks. Beyond that,**a 30-layer encoder is successfully trained**, resulting in a further improvement of 0.4 BLEU points. This is 0.6 BLEU points higher than the pre-norm Transformer-Big.

Although

the best score of 29.3 is the same as Ott et al. (2018), the proposed approachonly requires 3.5× fewer training epochsthan theirs.

- DLCL in both post-norm and pre-norm cases outperform Transparent Atenttion (TA) by Bapna et al. (2018).

## 3.2. Zh-En-Small Task

- Firstly DLCL is superior to the baseline when the network’s depth is shallow. Interestingly, both Transformer and DLCL achieve the best results when a 25-layer encoder is used.

## 3.3. Zh-En-Large Task

- The 25-layer pre-norm DLCL slightly surpassed Transformer-Big, and the superiority is bigger when using a 30-layer encoder.

## 3.4. Effect of Encoder Depth

- Remarkably, when the encoder depth reaches 20, both of the two deep models can achieve comparable performance to Transformer-Big.

Pre-norm Transformer degenerates earlier and is

less robust than DLCL when the depth is beyond 20.

The proposed system with a

30-layer encoder is still faster thanTransformer-Big.

## 3.5. Effect of Decoder Depth

Different from encoder, increasing the depth of decoder only yields a slight BLEU improvement.

## 3.6. Effect of DLCL

- Replacing learnable weights with constant weights:
**All-One**(*Wij*=1) and**Average**(*Wij*=1/(*i*+1))**consistently hurt performance**.

Making the weights learnable is important.

## 3.7. Weight Visualization

- The connections in the early layers are dense, but become sparse as the depth increases.
- Most of the large weight values concentrate on the right of the matrix, which indicates that
**the impact of the incoming layer is usually related to the distance between the outgoing layer, but the contribution to successive layers changes dynamically (one column)**.

## Reference

[2019 ACL] [Pre-Norm Transformer]

Learning Deep Transformer Models for Machine Translation

## Machine Translation

**2014** [Seq2Seq] [RNN Encoder-Decoder] **2015** [Attention Decoder/RNNSearch] **2016** [GNMT] [ByteNet] [Deep-ED & Deep-Att] **2017 **[ConvS2S] [Transformer] [MoE] [GMNMT] [CoVe] **2018 **[Shaw NAACL’18] **2019 **[AdaNorm] [GPT-2] [Pre-Norm Transformer] **2020 **[Batch Augment, BA] [GPT-3] [T5] **2021 **[ResMLP]