Neural machine translation (NMT) has achieved great success and drawn much attention in recent years Sutskever et al. (2014); Bahdanau et al. (2015); Vaswani et al. (2017). While conventional NMT can well handle the translation of a single language pair, training an individual model for each language pair is resource-consuming, considering there are thousands of languages in the world. Therefore, multilingual NMT is developed to handle multiple language pairs in one model, greatly reducing the cost of offline training and online deployment Ha et al. (2016); Johnson et al. (2017). Besides, the parameter sharing in multilingual neural machine translation encourages positive knowledge transfer among different languages and benefits low-resource translation Zhang et al. (2020); Siddhant et al. (2020).
Despite the benefits of the joint training with a completely shared model, the MNMT model also suffers from insufficient model capacity Arivazhagan et al. (2019); Lyu et al. (2020). The shared parameters tend to preserve the general knowledge but ignore language-specific knowledge. Therefore, researchers resort to heuristically design additional language-specific components and build MNMT model with a mix of shared and language-specific parameters to increase the model capacity Sachan and Neubig (2018); Wang et al. (2019b), such as the language-specific attention Blackwood et al. (2018), lightweight language adapter Bapna and Firat (2019) or language-specific routing layer Zhang et al. (2021). These methods simultaneously model the general knowledge and the language-specific knowledge but require specialized manual design. Another line of works for language-specific modeling aims to automatically search for language-specific sub-networks Xie et al. (2021); Lin et al. (2021), in which they pretrain an initial large model that covers all translation directions, followed by sub-network pruning and fine-tuning. These methods include multi-stage training and it is non-trivial to determine the initial model size and structure.
In this study, we propose a novel parameter differentiation based method that enables the model to automatically determine which parameters should be shared and which ones should be language-specific during training. Inspired by cellular differentiation, a process in which a cell changes from one general cell type to a more specialized type, our method allows each parameter that shared by multiple tasks to dynamically differentiate into more specialized types. As shown in Figure 1, the model is initialized as completely shared and continuously detects shared parameters that should be language-specific. These parameters are then duplicated and reallocated to different tasks to increase language-specific modeling capacity. The differentiation criterion is defined as inter-task gradient similarity, which represents the consistency of optimization direction across tasks on a shared parameter. Therefore, the parameters facing conflicting inter-task gradients are selected for differentiation while other parameters with more similar inter-task gradients remain shared. In general, the MNMT model in our method can gradually improve its parameter sharing configuration without multi-stage training or manually designed language-specific modules.
We conduct extensive experiments on three widely used multilingual datasets including OPUS, WMT and IWSLT in multiple MNMT scenarios: one-to-many, many-to-one and many-to-many translation. The experimental results prove the effectiveness of the proposed method over various strong baselines. Our main contributions can be summarized as follows:
We propose a method that can automatically determine which parameters in an MNMT model should be language-specific without manual design, and can dynamically change shared parameters into more specialized types.
We define the differentiation criterion as the inter-task gradient similarity, which helps to minimizes the inter-task interference on shared parameters.
We show that the parameter sharing configuration obtained by our method is highly correlated with linguistic features like language families.
The Transformer Model
A typical Transformer model Vaswani et al. (2017) consists of an encoder and a decoder. Both the encoder and the decoder are stacked with identical layers. Each encoder layer contains two modules named multi-head self-attention and feed-forward network. The decoder layer, containing three modules, inserts an additional multi-head cross-attention between the self-attention and feed-forward modules.
Multilingual Neural Machine Translation
The standard paradigm of MNMT contains a completely shared model borrowed from bilingual translation for all language pairs. A special language token is appended to the source text to indicate the target language, i.e., Johnson et al. (2017). The MNMT is often referred to as multi-task optimization, in which a task indicates a translation direction, e.g. ENDE.
3 Parameter Differentiation based MNMT
Our main idea is to find out shared parameters that should be language-specific in an MNMT model and dynamically change them into more specialized types during training. To achieve this, we propose a novel parameter differentiation based MNMT approach and define the differentiation criterion as inter-task gradient similarity.
3.1 Parameter Differentiation
As we know that cellular differentiation is the process in which a cell changes from one cell type to another, typically from a less specialized type (stem cell) to a more specialized type (organ/tissue-specific cell) Slack (2007). Inspired by cellular differentiation, we propose parameter differentiation that can dynamically change the task-agnostic parameters in an MNMT model into other task-specific types during training.
Algorithm 1 lists the overall process of our method. We first initialize the completely shared MNMT model following the paradigm in Johnson et al. (2017). After training for several steps, the model evaluates each shared parameter and flag the parameters that should become more specialized under a certain differentiation criterion (Line -). For those flagged parameters, the model then duplicates them and reallocates the replicas for different tasks. After the duplication and reallocation, the model builds new connections for those replicas to construct different computation graphs for each task (Line -). In the following training steps, the parameters belonging to only update for training data of task . The differentiation happens after every several training steps and the model dynamically becomes more specialized.
3.2 The Differentiation Criterion
The key issue in parameter differentiation is the definition of differentiation criterion that helps to detect the shared parameters that should differentiate into more specialized types. We define the differentiation criterion based on inter-task gradient cosine similarity, where the parameters facing conflicting gradients are more likely to be language-specific.
As shown in Figure 2, the parameter is shared by tasks , , and at the beginning. To determine whether the shared parameter should be more specialized, we first define the interference degree of the parameter shared by the three tasks with the inter-task gradient cosine similarity. More formally, suppose the -th parameter in an MNMT model is shared by a set of tasks , the interference degree of the parameter is defined by:
where and are the gradients of task and respectively on the parameter .
Intuitively, the gradients determine the optimization directions. For example in Figure 2, the gradient indicates the direction of global optimum for task . The gradients with maximum negative cosine similarity, such as and , point to opposite directions, which hinders the optimization and has been proved detrimental for multi-task learning Yu et al. (2020); Wang et al. (2021).
The gradients of each task on each shared parameter are evaluated on held-out validation data
. To minimize the gradient variance caused by inconsistent sentence semantics across languages, the validation data is created as multi-way aligned, i.e., each sentence has translations of all languages. With the held-out validation data, we evaluate gradients of each task on each shared parameter for calculating inter-task gradient similarities as well as the interference degreefor each parameter.
The interference degree helps the model to find out parameters that face severe interference and the parameters with high interference degrees are flagged for differentiation. Suppose the parameter shared by tasks is flagged, we cluster the tasks in into two subsets and that minimize the overall interference. The partition is obtained by:
As shown in Figure 2, the gradients of and are similar while and are in conflict with each other. By minimizing the overall interference degree, the tasks are clustered into partition . The parameter is then duplicated into and and the replicas are allocated to and respectively.
3.3 The Differentiation Granularity
|Granularity||Examples of Differentiation Units|
|Layers||encoder layer, decoder layer|
|Module||self-attention, feed-forward, cross-attention|
|Operation||linear projection, layer normalization|
In theory, each shared parameter can differentiate into more specialized types individually. But in practice, performing differentiation on every single parameter is resource- and time-consuming, considering there are millions to billions of parameters in an MNMT model.
Therefore, we resort to different levels of differentiation granularity, like Layer, Module, or Operation. As shown in Table 1
, the Layer granularity indicates different layers in the model, while the Module granularity specifies the individual modules within a layer. The Operation granularity includes the basic transformations in the model that contain trainable parameters. With a certain granularity, the parameters are grouped into different differentiation units. For example, with the Layer level granularity, the parameters within a layer are concatenated into a vector and differentiate together, where the vector is referred to as a differentiation unit. We list the differentiation units in supplementary materials.
In our method, since the model architecture dynamically changes and results in different computation graphs for each task, we create batches from the multilingual dataset and ensure that each batch contains only samples from one task. This is different from the training of vanilla completely shared MNMT model where each batch may contain sentence pairs from different languages Johnson et al. (2017). Specifically, we first sample a task , followed by sampling a batch from training data of . Then, the model which includes a mix of shared and language-specific parameters is trained with the batch .
We train the model with the Adam optimizer Kingma and Ba (2015), which computes adaptive learning rates based on the optimizing trajectory of past steps. However, the optimization history becomes inaccurate for the differentiated parameters. For the example in Figure 2, the differentiated parameter is only shared by task , while the optimization history of represents the optimizing trajectory of all the tasks. To stabilize the training of on task , we reinitialize the optimizer states by performing a warm-up update for those differentiated parameters:
where and are the Adam states of , and is the gradient of task on the held-out validation data. Note that we only update the states in the Adam optimizer and the parameters remain unchanged in the warm-up update step.
We use the public OPUS and WMT multilingual datasets to evaluate our method on many-to-one (M2O) and one-to-many (O2M) translation scenarios, and the IWSLT datasets for the many-to-many (M2M) translation scenario.
The OPUS dataset consists of English to languages selected from the original OPUS-100 dataset Zhang et al. (2020). These languages, containing M sentences for each, are from distinct language groups: Romance (French, Italian), Baltic (Latvian, Lithuanian), Uralic (Estonian, Finnish), Austronesian (Indonesian, Malay), West-Slavic (Polish, Czech) and East-Slavic (Ukrainian, Russian).
The WMT dataset with unbalanced data distribution is collected from the WMT’14, WMT’16 and WMT’18 benchmarks. We select languages with data sizes ranging from M to M. The training data sizes and sources are shown in Table 2. We report the results on the WMT dataset with the temperature-based sampling in which the temperature is set to Arivazhagan et al. (2019).
We evaluate our method on the many-to-many scenario with the IWSLT’17 dataset, which includes German, English, Italian, Romanian, and Dutch, and results in translation directions between the languages. Each translation direction contains about k sentence pairs.
The held-out multi-way aligned validation data for measuring gradient similarities contains sentences for each language, and are randomly selected and excluded from the training set. We apply the byte-pair encoding (BPE) algorithm Sennrich et al. (2016) with vocabulary sizes of k for both OPUS and WMT datasets, and k for the IWSLT dataset.
|Language Pair||Data Source||#Samples|
|Baselines||Bilingual Vaswani et al. (2017)||28.90||28.27||22.55||25.55||31.60||39.75||28.88||36.43||18.65||25.48|
|Multilingual Johnson et al. (2017)||27.33||28.31||21.20||27.09||30.00||40.10||27.69||37.15||20.08||30.09|
|Tan et al. (2019)||27.39||29.21||21.97||26.77||31.85||42.71||29.27||39.34||21.40||29.79|
|Sachan and Neubig (2018)||28.04||29.31||22.86||27.86||32.04||41.43||28.47||38.14||21.41||30.30|
|Ours||PD w. Layer||29.35||30.09||22.37||28.7||32.31||42.11||29.5||39.04||20.56||30.91|
|PD w. Module||29.09||30.09||22.49||28.64||31.86||41.60||29.53||39.04||21.25||31.11|
|PD w. Operation||29.26||30.11||23.01||28.6||33.06||42.38||29.94||39.54||20.89||31.14|
|Baselines||Bilingual Vaswani et al. (2017)||13.92||18.34||21.29||25.61||16.75||21.24||13.46||19.05||16.82||25.27|
|Multilingual Johnson et al. (2017)||15.58||21.43||22.85||28.27||18.12||23.66||14.87||22.24||18.57||28.14|
|Tan et al. (2019)||16.15||21.46||22.74||28.00||18.12||23.14||14.86||21.72||18.02||28.08|
|Sachan and Neubig (2018)||16.37||21.36||22.39||29.60||17.33||23.77||15.75||22.45||19.70||28.59|
|Ours||PD w. Layer||16.42||22.37||22.89||29.28||18.35||24.88||16.07||23.11||19.29||29.31|
|PD w. Module||16.44||22.85||22.94||28.86||17.62||24.27||16.18||23.12||19.33||29.08|
|PD w. Operation||16.59||22.85||23.09||29.03||18.61||25.27||16.45||23.34||19.46||29.66|
|Baselines||Bilingual Vaswani et al. (2017)||10.06||18.68||21.63||26.61||20.38||25.86||-0.36||-2.22||12x||12x|
|Multilingual Johnson et al. (2017)||11.59||21.76||20.96||28.76||20.74||28.08||0||0||1x||1x|
|Tan et al. (2019)||11.32||21.74||21.32||28.73||21.20||28.39||+0.46||+0.31||2x||2x|
|Sachan and Neubig (2018)||10.96||21.88||22.28||28.80||21.47||28.62||+0.73||+0.54||3.71x||3.25x|
|Ours||PD w. Layer||12.32||22.68||22.82||30.37||21.85||29.40||+1.11||+1.32||2.14x||1.84x|
|PD w. Module||12.55||22.44||22.31||30.39||21.80||29.29||+1.06||+1.21||1.82x||1.94x|
|PD w. Operation||12.37||23.05||22.98||30.60||22.14||29.63||+1.40||+1.55||1.96x||1.90x|
4.2 Model Settings
We conduct our experiments with the Transformer architecture and adopt the transformer_base setting which includes encoder and decoder layers, / hidden dimensions and attention heads. Dropout () and label smoothing () are applied during training but disabled during validation and inference. Each mini-batch contains roughly tokens. We accumulate gradients and update the model every steps for OPUS and steps for WMT to simulate multi-GPU training. In inference, we use beam search with the beam size of and the length penalty of . We measure the translation quality by BLEU score Papineni et al. (2002) with SacreBLEU111https://github.com/mjpost/sacrebleu. All the models are trained and tested on a single Nvidia V100 GPU.
Our method allows the parameters to differentiate into specialized types by duplication and reallocation, which may results in bilingual models with unlimited parameter differentiation, i.e., each parameter is only shared by one task in the final model. To prevent over-specialization and make a fair comparison, we set a differentiation upper bound defined by the expected final model size , and let the model control the number of parameters (denoted as ) to differentiate222 Since the parameters are grouped into differentiation units under a certain granularity, the value of and may fluctuate to comply with the granularity.:
where is the size of the original completely shared model. The total training step is set to k for all experiments, and the differentiation happens every steps of training. We set the expected model size to , times of original model. We also analyze the relationship between model size and translation quality by varying in the range from to .
4.3 Baseline Systems
We compare our method with several baseline methods with different paradigms of parameter sharing.
Bilingual trains Transformer model Vaswani et al. (2017) for each translation direction and results in individual models for translation directions.
Multilingual adopts the standard paradigm of MNMT that all parameters are shared across tasks Johnson et al. (2017).
Random Sharing selects parameters for differentiation randomly (with Operation granularity) instead of using inter-task gradient similarity.
Sachan and Neubig (2018) uses a partially shared model that proved effective empirically. They share the attention key and query of the decoder, the embedding, and the encoder in a one-to-many model. We extend the settings for the many-to-one model that share the attention key and query of the encoder, the embedding, and the decoder.
Tan et al. (2019) first clusters the languages using the language embedding vectors in the Multilingual method and then trains one model for each cluster. To make the model size comparable with our method, we set the number of clusters as and train two distinct models. In our experiment on the OPUS dataset, this method results in two clusters: FR, IT, ID, MS, PL, CS, UK, RU and LV, LT, ET, FI.
|DE||-||33.09 / 34.07||20.10 / 21.05||22.18 / 22.35||18.20 / 19.00||23.39 / 24.12|
|EN||26.55 / 27.86||-||27.74 / 28.69||28.15 / 28.61||25.34 / 26.58||26.95 / 27.94|
|IT||19.32 / 20.37||32.14 / 32.99||-||20.05 / 20.47||19.28 / 20.10||22.70 / 23.48|
|NL||21.06 / 22.09||32.54 / 33.45||19.81 / 20.64||-||17.93 / 18.81||22.84 / 23.75|
|RO||20.74 / 21.39||34.78 / 35.76||22.96 / 23.53||20.87 / 21.04||-||24.84 / 25.43|
|Average||21.92 / 22.93||33.14 / 34.07||22.65 / 23.48||22.81 / 23.12||20.19 / 21.12||30.08 / 31.18|
OPUS Table 3 shows the results of our method and the baseline methods on the OPUS dataset. In both one-to-many () and many-to-one () directions, our methods consistently outperform the Bilingual and Multilingual baselines and gains improvement over the Multilingual baseline by up to and BLEU on average. Compared to other parameter sharing methods, our method achieves the best results in of translation directions and improves the average BLEU by a large margin. As for the different granularities in our method, we find that the Operation level achieves the best results on average, due to the fine-grained control of parameter differentiation compared to the Layer level and the Module level.
For the model sizes, the method of Sachan and Neubig (2018) that pre-defines the sharing modules increases linearly with the number of languages involved and results in a larger model size (x). In our method, the model size is unrelated to the number of languages, which provides more scalability and flexibility. Since we use different granularities instead of performing differentiation on every single parameter, the actual sizes of our method range from x to x, close but not equal to the predefined x.
WMT We further investigate the generalization performance with experiments on the unbalanced WMT dataset. As shown in Table 4, the Multilingual model benefits lower-resource languages (ET, RO) translation but hurts the performances of higher-resource languages (FR, CS, DE). In contrast, our method gains more improvements in higher-resource language (+2.21 for FREN) than lower-resource language (+1.05 for ROEN). Our method can also outperform the Bilingual method in of translation directions.
IWSLT The results on the many-to-many translation scenario with the IWSLT dataset are shown in Table 5. Our method based on Operation level granularity outperforms the Multilingual baseline in all translation directions, but the improvement ( BLEU on average) is less significant than those on the other two datasets. The reason is that the languages in the IWSLT dataset belong to the same Indo-European language family and thus the shared parameters may be sufficient for modeling all translation directions.
Parameter Differentiation Across Layers
Using a shared encoder for one-to-many translation and a shared decoder for many-to-one translation has been proved effective and is widely used Zoph and Knight (2016); Dong et al. (2015); Sachan and Neubig (2018). However, there lack of analyses on different sharing strategies across layers. The parameter differentiation method provides a more fine-grained control of parameter sharing, making it possible to offer such analyses. To investigate the parameter sharing across layers, we calculate the number of differentiation units within each layer of the final model trained with Operation level granularity. For comparison, the completely shared model has differentiation units in each encoder layer and in each decoder layer (see details in supplementary materials).
The results are shown in Figure 3. For many-to-one translation, the task-specific parameters are mainly distributed in shallower layers of the encoder and the parameters in the decoder tend to stay shared. On contrary, for one-to-many translation, the decoder has more task-specific parameters than the encoder. Different from the encoder in which shallower layers are slightly more task-specific, both the shallower and the deeper layers are more specific than the middle layers in the decoder. The reason is that the shallower layers in the decoder take tokens from multiple languages as input and the deeper layers are responsible for generating tokens in multiple languages.
Parameter Differentiation and Language Family
We investigate the correlation between the parameter sharing obtained by differentiation and the language families. Intuitively, linguistically similar languages are more likely to have shared parameters. To verify this, we first select encoder.layer-0.self-attention.value-projection, which differentiate for the most times and is the most specialized, and then analyze its differentiation process during training.
Figure 4 shows the differentiation process of the most specialized parameter. From the training steps, we can find that the differentiation happens aperiodically for this parameter. As for the differentiation results, it is obvious that the parameter sharing strategy is highly correlated with the linguistic proximity like language family or language branch. For example, ID and MS belong to the Austronesian language and share the parameters while ID and FR belonging to the Austronesian language and the Romance language respectively have task-specific parameters. Another interesting observation is that the Baltic languages (LV and LT) become specialized at the early stage of training. We examine the OPUS dataset and find out that the training data of LV and LT are mainly from the political domain, while other languages are mainly from the spoken domain.
The Effect of Model Size
We notice that the model size is not completely correlated with performance according to the results in Table 3. Our method initialize the model as completely shared with the model size of x, and may differentiate into bilingual models in extreme cases. The completely shared model tends to preserve the general knowledge, while the bilingual models only capture language-specific knowledge. To investigate the effect of the differentiation level, we evaluate the relationship between model size and translation quality.
As shown in Figure 5, the performance first increases with a higher differentiation level (larger model size) and then decreases when the model grows over a certain threshold. The best results are obtained with x and x model sizes for one-to-many and many-to-one directions respectively, which indicates that the model needs more parameters for handling multiple target languages (one-to-many) than multiple source languages (many-to-one).
5 Related Work
Multilingual neural machine translation (MNMT) aims at handling translation between multiple languages with a single model Dabre et al. (2020). In the early stage, researchers share different modules like encoder Dong et al. (2015), decoder Zoph and Knight (2016), or attention mechanism Firat et al. (2016) to reduce the parameter scales in bilingual models. The success in sharing modules motivates a more aggressive parameter sharing that handles all languages with a completely shared model Johnson et al. (2017); Ha et al. (2016).
Despite its simplicity, the completely shared model faces capacity bottlenecks for retaining specific knowledge of each language Aharoni et al. (2019). Researchers resort to language specific modeling with various parameter sharing strategies Sachan and Neubig (2018); Wang et al. (2019b, 2018), such as the attention module Wang et al. (2019a); Blackwood et al. (2018); He et al. (2021), decoupling encoder or decoder Escolano et al. (2021), additional adapters Bapna and Firat (2019), and language clustering Tan et al. (2019).
Instead of augmenting the model with manually designed language-specific modules, researchers attempt to search for a language-specific sub-space of the model, such as generating the language-specific parameters from global ones Platanios et al. (2018), language-aware model depth Li et al. (2020), language-specific routing path Zhang et al. (2021) and language-specific sub-networks Xie et al. (2021); Lin et al. (2021). These methods start from a large model that covers all translation directions, where the size and structure of the initial model are non-trivial to determine. While our method initializes a simple shared model and lets the model to automatically grows into a more complicated one, which provides more scalability and flexibility.
6 Conclusion and Future Work
In this paper, we propose a novel parameter differentiation based method that can automatically determine which parameters should be shared and which ones should be language-specific. The shared parameters can dynamically differentiate into more specialized types during training. The extensive experiments on three multilingual machine translation datasets verify the effectiveness of our method. The analyses reveal that the parameter sharing configurations obtained by our method are highly correlated with the linguistic proximities. In the future, we want to let the model learn when to stop differentiation and explore other differentiation criteria for more multilingual scenarios like the zero-shot translation and the incremental multilingual translation.
The research work descried in this paper has been supported by the Natural Science Foundation of China under Grant No. U1836221 and 62122088, and also by Beijing Academy of Artificial Intelligence (BAAI).
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