A Stack-Propagation Framework with Token-Level Intent Detection for Spoken Language Understanding

09/05/2019 ∙ by Libo Qin, et al. ∙ Harbin Institute of Technology 0

Intent detection and slot filling are two main tasks for building a spoken language understanding (SLU) system. The two tasks are closely tied and the slots often highly depend on the intent. In this paper, we propose a novel framework for SLU to better incorporate the intent information, which further guides the slot filling. In our framework, we adopt a joint model with Stack-Propagation which can directly use the intent information as input for slot filling, thus to capture the intent semantic knowledge. In addition, to further alleviate the error propagation, we perform the token-level intent detection for the Stack-Propagation framework. Experiments on two publicly datasets show that our model achieves the state-of-the-art performance and outperforms other previous methods by a large margin. Finally, we use the Bidirectional Encoder Representation from Transformer (BERT) model in our framework, which further boost our performance in SLU task.

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1 Introduction

Spoken language understanding (SLU) is a critical component in task-oriented dialogue systems. It usually consists of intent detection to identify users’ intents and slot filling task to extract semantic constituents from the natural language utterances Tur and De Mori (2011). As shown in Table 1, given a movie-related utterance “watch action movie”, there are different slot labels for each token and an intent for the whole utterance.

Usually, intent detection and slot filling are implemented separately. But intuitively, these two tasks are not independent and the slots often highly depend on the intent Goo et al. (2018). For example, if the intent of a utterance is WatchMovie, it is more likely to contain the slot movie_name rather than the slot music_name. Hence, it is promising to incorporate the intent information to guide the slot filling.

width=0.48

Sentence watch action movie
Gold Slots O B-movie_name I-movie_name
Gold Intent WatchMovie
Table 1: An example with intent and slot annotation (BIO format), which indicates the slot of movie name from an utterance with an intent WatchMovie.

Considering this strong correlation between the two tasks, some joint models are proposed based on the multi-task learning framework Zhang and Wang (2016); Hakkani-Tür et al. (2016); Liu and Lane (2016) and all these models outperform the pipeline models via mutual enhancement between two tasks. However, their work just modeled the relationship between intent and slots by sharing parameters. Recently, some work begins to model the intent information for slot filling explicitly in joint model. Goo et al. (2018) and Li et al. (2018)

proposed the gate mechanism to explore incorporating the intent information for slot filling. Though achieving the promising performance, their models still suffer from two issues including: (1) They all adopt the gate vector to incorporate the intent information. In the paper, we argue that it is risky to simply rely on the gate function to summarize or memorize the intent information. Besides, the interpretability of how the intent information guides slot filling procedure is still weak due to the interaction with hidden vector between the two tasks. (2) The utterance-level intent information they use for slot filling may mislead the prediction for all slots in an utterance if the predicted utterance-level intent is incorrect.

Figure 1: Multi-task framework vs. Stack-Propagation.

In this paper, we propose a novel framework to address both two issues above. For the first issue, inspired by the Stack-Propagation which was proposed by P16-1147 to leverage the POS tagging features for parsing and achieved good performance, we propose a joint model with Stack-Propagation for SLU tasks. Our framework directly use the output of the intent detection as the input for slot filling to better guide the slot prediction process. In addition, the framework make it easy to design oracle intent experiment to intuitively show how intent information enhances slot filling task. For the second issue, we perform a token-level intent prediction in our framework, which can provide the token-level intent information for slot filling. If some token-level intents in the utterance are predicted incorrectly, other correct token-level intents will still be useful for the corresponding slot prediction. In practice, we use a self-attentive encoder for intent detection to capture the contextual information at each token and hence predict an intent label at each token. The intent of an utterance is computed by voting from predictions at each token of the utterance. This token-level prediction, like ensemble neural networks

Lee et al. (2016)

, reduces the predicted variance to improve the performance of intent detection. And it fits better in our Stack-Propagation framework, where intent detection can provide token-level intent features and retain more useful intent information for slot filling.

We conduct experiments on two benchmarks SNIPS Coucke et al. (2018) and ATIS Goo et al. (2018) datasets. The results of both experiments show the effectiveness of our framework by outperforming the current state-of-the-art methods by a large margin. Finally, Bidirectional Encoder Representation from Transformer (Devlin et al., 2018, BERT), as the pre-trained model, is used to further boost the performance of our model.

To summarize, the contributions of this work are as follows:

  • We propose a Stack-Propagation framework in SLU task, which can better incorporate the intent semantic knowledge to guide the slot filling and make our joint model more interpretable.

  • We perform the token-level intent detection for Stack-Propagation framework, which improves the intent detection performance and further alleviate the error propagation.

  • We present extensive experiments demonstrating the benefit of our proposed framework. Our experiments on two publicly available datasets show substantial improvement and our framework achieve the state-of-the-art performance.

  • We explore and analyze the effect of incorporating BERT in SLU tasks.

For reproducibility, our code for this paper is publicly available at https://github.com/LeePleased/StackPropagation-SLU.

2 Background

In this section, we will describe the formulation definition for intent detection and slot filling, and then we give a brief description of the multi-task framework and the joint model with Stack-Propagation framework.

2.1 Intent Detection and Slot Filling

Intent detection can be seen as a classification problem to decide the intent label of an utterance. Slot filling is a sequence labeling task that maps an input word sequence = to slots sequence = .

2.2 Multi-task Framework vs. Stack-Propagation

For two correlative tasks Task A and Task B, multi-task framework which is shown in Figure 1 (a) can learn the correlations between these two tasks by the shared encoder. However, the basic multi-task framework cannot provide features from up-stream task to down-stream task explicitly. The joint model with Stack-Propagation framework which is shown in Figure 1 (b) can mitigate the shortcoming. In this figure, Task B can leverage features of Task A without breaking the differentiability in Stack-Propagation framework and promote each other by joint learning at the same time.

Figure 2: Illustration of our Stack-Propagation framework for joint intent detection and slot filling. It consists of one shared self-attentive encoder and two decoders. The output distribution of intent detection network and the representations from encoder are concatenated as the input for slot filling.

3 Approach

In this section, we will describe our Stack-Propagation framework for SLU task. The architecture of our framework is demonstrated in Figure 2, which consists of an encoder and two decoders. First, the encoder module uses one shared self-attentive encoder to represent an utterance, which can grasp the shared knowledge between two tasks. Then, the intent-detection decoder performs a token-level intent detection. Finally, our Stack-Propagation framework leverages the explicit token-level intent information for slot filling by concatenating the output of intent detection decoder and the representations from encoder as the input for slot filling decoder. Both intent detection and slot filling are optimized simultaneously via a joint learning scheme.

3.1 Self-Attentive Encoder

In our Stack-Propagation framework, intent detection task and slot filling task share one encoder, In the self-attentive encoder, we use BiLSTM with self-attention mechanism to leverage both advantages of temporal features and contextual information, which are useful for sequence labeling tasks Zhong et al. (2018); Yin et al. (2018).

The BiLSTM Hochreiter and Schmidhuber (1997) reads input utterance = () ( is the number of tokens in the input utterance) forwardly and backwardly to produce context-sensitive hidden states = by repeatedly applying the recurrence .

Self-attention is a very effective method of leveraging context-aware features over variable-length sequences for natural language processing tasks

Tan et al. (2018); Zhong et al. (2018). In our case, we use self-attention mechanism to capture the contextual information for each token. In this paper, we adopt Vaswani et al. (2017), where we first map the matrix of input vectors ( represents the mapped dimension) to queries (), keys () and values () matrices by using different linear projections and the self-attention output C is a weighted sum of values:

(1)

After obtaining the output of self-attention and BiLSTM. We concatenate these two representations as the final encoding representation:

(2)

where E and is concatenation operation.

3.2 Token-Level Intent Detection Decoder

In our framework, we perform a token-level intent detection, which can provide token-level intent features for slot filling, different from regarding the intent detection task as the sentence-level classification problem Liu and Lane (2016). The token-level intent detection method can be formalized as a sequence labeling problem that maps a input word sequence = to sequence of intent label = . In training time, we set the sentence’s intent label as every token’s gold intent label. The final intent of an utterance is computed by voting from predictions at each token of the utterance.

The self-attentive encoder generates a sequence of contextual representations and each token can grasp the whole contextual information by self-attention mechanism. We use a unidirectional LSTM as the intent detection network. At each decoding step , the decoder state is calculated by previous decoder state , the previous emitted intent label distribution and the aligned encoder hidden state :

(3)

Then the decoder state is utilized for intent detection:

(4)
(5)

where is the intent output distribution of the th token in the utterance; represents the intent lable of th token and are trainable parameters of the model.

The final utterance result is generated by voting from all token intent results:

(6)

where is the length of utterance and is the number of intent labels; denotes a 0-1 vector of which the th unit is one and the others are zero; indicates the operation returning the indices of the maximum values in .

By performing the token-level intent detection, there are mainly two advantages:

1. Performing the token-level intent detection can provide features at each token to slot filling in our Stack-Propagation framework, which can ease the error propagation and retain more useful information for slot filling. Compared with sentence-level intent detection Li et al. (2018), if the intent of the whole sentence is predicted wrongly, the wrong intent would possibly apply a negative impact on all slots. However, in token-level intent detection, if some tokens in the utterance predicted wrongly, other correct token-level intent information will still be useful for the corresponding slot filling.

2. Since each token can grasp the whole utterance contextual information by using the self-attentive encoder, we can consider predictions at each token in an utterance as individual prediction to the intent of this utterance. And therefore, like ensemble neural networks, this approach will reduce the predicted variance and improve the performance of intent detection. The experiments section empirically demonstrate the effectiveness of the token-level intent detection.

3.3 Stack-propagation for Slot Filling

In this paper, one of the advantages of our Stack-Propagation framework is directly leveraging the explicit intent information to constrain the slots into a specific intent and alleviate the burden of slot filling decoder. In our framework, we compose the input units for slot filling decoder by concatenating the intent output distribution and the aligned encoder hidden state .

For the slot-filling decoder, we similarly use another unidirectional LSTM as the slot-filling decoder. At the decoding step , the decoder state can be formalized as:

(7)

where is the previous decoder state; is the previous emitted slot label distribution.

Similarily, the decoder state is utilized for slot filling:

(8)
(9)

where is the slot label of the th word in the utterance.

3.4 Joint Training

Another major difference between existing joint work Zhang and Wang (2016); Goo et al. (2018) and our framework is the training method for intent detection, where we convert the sentence-level classification task into token-level prediction to directly leverage token-level intent information for slot filling. And the intent detection objection is formulated as:

(10)

Similarly, the slot filling task objection is defined as:

(11)

where and are the gold intent label and gold slot label separately; is the number of slot labels.

To obtain both slot filling and intent detection jointly, the final joint objective is formulated as

(12)

Through the joint loss function, the shared representations learned by the shared self-attentive encoder can consider two tasks jointly and further ease the error propagation compared with pipeline models

Zhang and Wang (2016).

4 Experiments

4.1 Experimental Settings

To evaluate the efficiency of our proposed model, we conduct experiments on two benchmark datasets. One is the publicly ATIS dataset Hemphill et al. (1990) containing audio recordings of flight reservations, and the other is the custom-intent-engines collected by Snips (SNIPS dataset) Coucke et al. (2018). 111https://github.com/snipsco/nlu-benchmark/tree/master/2017-06-custom-intent-engines Both datasets used in our paper follows the same format and partition as in goo2018slot. The dimensionalities of the word embedding is 256 for ATIS dataset and 512 for SNIPS dataset. The self-attentive encoder hidden units are set as 256. L2 regularization is used on our model is and dropout ratio is adopted is 0.4 for reducing overfit. We use Adam Kingma and Ba (2014) to optimize the parameters in our model and adopted the suggested hyper-parameters for optimization. For all the experiments, we select the model which works the best on the dev set, and then evaluate it on the test set.

4.2 Baselines

We compare our model with the existing baselines including:

  • Joint Seq

    . hakkani2016multi proposed a multi-task modeling approach for jointly modeling domain detection, intent detection, and slot filling in a single recurrent neural network (RNN) architecture.

  • Attention BiRNN. liu2016attention leveraged the attention mechanism to allow the network to learn the relationship between slot and intent.

  • Slot-Gated Atten. goo2018slot proposed the slot-gated joint model to explore the correlation of slot filling and intent detection better.

  • Self-Attentive Model

    . li2018self proposed a novel self-attentive model with the intent augmented gate mechanism to utilize the semantic correlation between slot and intent.

  • Bi-Model. wang2018bi proposed the Bi-model to consider the intent and slot filling cross-impact to each other.

  • CAPSULE-NLU. zhang-etal-2019-joint proposed a capsule-based neural network model with a dynamic routing-by-agreement schema to accomplish slot filling and intent detection.

  • SF-ID Network. E et al. (2019) introduced an SF-ID network to establish direct connections for the slot filling and intent detection to help them promote each other mutually.

For the Joint Seq, Attention BiRNN, Slot-gated Atten, CAPSULE-NLU and SF-ID Network, we adopt the reported results from goo2018slot, zhang-etal-2019-joint,e-etal-2019-novel. For the Self-Attentive Model, Bi-Model, we re-implemented the models and obtained the results on the same datasets.222All experiments are conducted on the publicly datasets provided by goo2018slot, Self-Attentive Model and Bi-Model don’t have the reported result on the same datasets or they did different preprocessing. For directly comparison, we re-implemented the models and obtained the results on the ATIS and SNIPS datasets preprocessed by Goo et al. (2018). Because all baselines and our model don’t apply CRF layer, we just report the best performance of SF-ID Network without CRF. It’s noticing that our model does outperform SF-ID Network with CRF layer.

Model SNIPS ATIS
Slot (F1) Intent (Acc) Overall (Acc) Slot (F1) Intent (Acc) Overall (Acc)
Joint Seq Hakkani-Tür et al. (2016) 87.3 96.9 73.2 94.3 92.6 80.7
Attention BiRNN Liu and Lane (2016) 87.8 96.7 74.1 94.2 91.1 78.9
Slot-Gated Full Atten Goo et al. (2018) 88.8 97.0 75.5 94.8 93.6 82.2
Slot-Gated Intent Atten Goo et al. (2018) 88.3 96.8 74.6 95.2 94.1 82.6
Self-Attentive Model Li et al. (2018) 90.0 97.5 81.0 95.1 96.8 82.2
Bi-Model Wang et al. (2018) 93.5 97.2 83.8 95.5 96.4 85.7
CAPSULE-NLU Zhang et al. (2019) 91.8 97.3 80.9 95.2 95.0 83.4
SF-ID Network E et al. (2019) 90.5 97.0 78.4 95.6 96.6 86.0
Our model 94.2* 98.0* 86.9* 95.9* 96.9* 86.5*
Oracle (Intent) 96.1 - - 96.0 - -
Table 2: Slot filling and intent detection results on two datasets. The numbers with * indicate that the improvement of our model over all baselines is statistically significant with

under t-test.

4.3 Overall Results

Following goo2018slot, we evaluate the SLU performance of slot filling using F1 score and the performance of intent prediction using accuracy, and sentence-level semantic frame parsing using overall accuracy. Table 2 shows the experiment results of the proposed models on SNIPS and ATIS datasets.

From the table, we can see that our model significantly outperforms all the baselines by a large margin and achieves the state-of-the-art performance. In the SNIPS dataset, compared with the best prior joint work Bi-Model, we achieve 0.7% improvement on Slot (F1) score, 0.8% improvement on Intent (Acc) and 3.1% improvement on Overall (Acc). In the ATIS dataset, we achieve 0.4% improvement on Slot (F1) score, 0.5% improvement on Intent (Acc) and 0.8% improvement on Overall (Acc). This indicates the effectiveness of our Stack-Propagation framework. Especially, our framework gains the largest improvements on sentence-level semantic frame accuracy, we attribute this to the fact that our framework directly take the explicit intent information into consideration can better help grasp the relationship between the intent and slots and improve the SLU performance.

To see the role of intent information for SLU tasks intuitively, we also present the result when using the gold intent information.333During the inference time, we concatenate the gold intent label distribution (one-hot vector) and the aligned encoder hidden state as the composed input for slot filling decoder. To keep the train and test procedure as the same, we replace our intent distribution as one-hot intent information for slot filling when train our model in our oracle experiments setting. The result is shown in the oracle row of Table 2. From the result, we can see that further leveraging better intent information will lead to better slot filling performance. The result also verifies our assumptions that intent information can be used for guiding the slots prediction.

4.4 Analysis

In Section 4.3, significant improvements among all three metrics have been witnessed on both two publicly datasets. However, we would like to know the reason for the improvement. In this section, we first explore the effect of Stack-Propagation framework. Next, we study the effect of our proposed token-level intent detection mechanism. Finally, we study the effect of self-attention mechanism in our framework.

Model SNIPS ATIS
Slot (F1) Intent (Acc) Overall (Acc) Slot (F1) Intent (Acc) Overall (Acc)
gate-mechanism 92.2 97.6 82.4 95.3 96.2 83.4
pipelined model 90.8 97.6 81.8 95.1 96.1 82.3
sentence intent augmented 93.7 97.5 86.1 95.5 96.7 85.8
lstm+last-hidden - 97.1 - - 95.2 -
lstm+token-level - 97.5 - - 96.0 -
without self-attention 94.1 97.8 86.6 95.6 96.6 86.2
Our model 94.2 98.0 86.9 95.9 96.9 86.5
Table 3: The SLU performance on baseline models compared with our Stack-Propagation model on two datasets.

4.4.1 Effect of Stack-Propagation Framework

To verify the effectiveness of the Stack-Propagation framework. we conduct experiments with the following ablations:

1) We conduct experiments to incorporate intent information by using gate-mechanism which is similar to goo2018slot, providing the intent information by interacting with the slot filling decoder by gate function.444For directly comparison, we still perform the token-level intent detection. We refer it as gate-mechanism.

2) We conduct experiments on the pipelined model where the intent detection and slot filling has their own self-attentive encoder separately. The other model components keep the same as our framework. We name it as pipelined model.

Table 3 gives the result of the comparison experiment. From the result of gate-mechanism row, we can observe that without the Stack-Propagation learning and simply using the gate-mechanism to incorporate the intent information, the slot filling (F1) performance drops significantly, which demonstrates that directly leverage the intent information with Stack-Propagation can improve the slot filling performance effectively than using the gate mechanism. Besides, we can see that the intent detection (Acc) and overall accuracy (Acc) decrease a lot. We attribute it to the fact that the bad slot filling performance harms the intent detection and the whole sentence semantic performance due to the joint learning scheme.

Besides, from the pipeline model row of Table 3, we can see that without shared encoder, the performance on all metrics declines significantly. This shows that Stack-Propagation model can learn the correlation knowledge which may promote each other and ease the error propagation effectively.

4.4.2 Effect of Token-Level Intent Detection Mechanism

In this section, we study the effect of the proposed token-level intent detection with the following ablations:

1) We conduct the sentence-level intent detection in intent detection separately, which utilizes the last hidden vector of BiLSTM encoder for intent detection. We refer it to lstm + last-hidden. For comparison, our token-level intent detection without joint learning with slot filling is named as lstm+token-level in Table 3.

2) We conduct a joint learning framework that slot filling uses the utterance-level intent information rather than token-level intent information for each token, similar to intent-gated mechanism Li et al. (2018), which is named as sentence intent augmented.

We show these two comparison experiments results in the first block of table 3. From the result, we can see that the token-level intent detection obtains better performance than the utterance-level intent detection. We believe the reason is that intent prediction on each token has similar advantage to ensemble neural networks, which can reduce the predicted variance to improve the intent performance. As a result, our framework can provide more useful intent information for slot filling by introducing token-level intent detection.

In addition, we can observe that if we only provide the sentence-level intent information for slot filling decoder, we obtain the worse results, which demonstrates the significance and effectiveness of incorporating token-level intent information. The main reason for this can be that incorporating the token-level intent information can retain useful features for each token and ease the error propagation.

4.4.3 Effect of Self-attention Mechanism

We further investigate the benefits of self-attention mechanism in our framework. We conduct the comparison experiments with the same framework except the self-attention encoder is replaced with BiLSTM.

Results are shown in the without self-attention row of Table 3. We can observe the self-attention mechanism can further improve the SLU performance. We attribute this to the fact that self-attention mechanism can capture the contextual information for each token. Without the self-attention mechanism, it will harm the intent detection and have bad influence on slot filling task by joint learning.

It is noticeable that even without the self-attention mechanism, our framework still performs the state-of-the-art Bi-model model Li et al. (2018), which again demonstrates the effectiveness and robustness of our other framework components.

Model SNIPS ATIS
Slot (F1) Intent (Acc) Overall (Acc) Slot (F1) Intent (Acc) Overall (Acc)
Our model 94.2 98.0 86.9 95.9 96.9 86.5
Intent detection (BERT) - 97.8 - - 96.5 -
Slot filling (BERT) 95.8 - - 95.6 - -
BERT SLU Chen et al. (2019) 97.0 98.6 92.8 96.1 97.5 88.2
Our model + BERT 97.0 99.0 92.9 96.1 97.5 88.6
Table 4: The SLU performance on BERT-based model on two datasets.

4.5 Effect of BERT

Finally, we also conduct experiments to use pre-trained model, BERT Devlin et al. (2018), to boost SLU performance. In this section, we replace the self-attentive encoder by BERT base model with the fine-tuning approach and keep other components as same with our framework.

Table 4 gives the results of BERT model on ATIS and SNIPS datasets. From the table, the BERT model performs remarkably well on both two datasets and achieves a new state-of-the-art performance, which indicates the effectiveness of a strong pre-trained model in SLU tasks. We attribute this to the fact that pre-trained models can provide rich semantic features, which can help to improve the performance on SLU tasks. In addition, our model + BERT outperforms the BERT SLU Chen et al. (2019) which apply BERT for joint the two tasks and there is no explicit interaction between intent detection and slot filling in two datasets in overall acc metric. It demonstrates that our framework is effective with BERT.

Especially, we also conduct experiments of intent detection task and slot filling separately based on BERT model. For intent detection, we put the special [CLS]

word embedding into a classification layer to classify the intent. For slot filling, we feed the final hidden representation

for each token 555We only consider the first subword label if a word is broken into multiple subwords into a classification layer over the slot tag set. The results are also shown in the Table 4. From the result, we can see that the slot filling (F1) and intent detection accuracy (Acc) is lower than our joint model based on BERT, which again demonstrates the effectiveness of exploiting the relationship between these two tasks.

5 Related Work

Slot filling can be treated as a sequence labeling task, and the popular approaches are conditional random fields (CRF) Raymond and Riccardi (2007) and recurrent neural networks (RNN) Xu and Sarikaya (2013); Yao et al. (2014)

. The intent detection is formulated as an utterance classification problem, and different classification methods, such as support vector machine (SVM) and RNN

Haffner et al. (2003); Sarikaya et al. (2011), has proposed to solve it.

Recently, there are some joint models to overcome the error propagation caused by the pipelined approaches. zhang2016joint first proposed the joint work using RNNs for learning the correlation between intent and slots. hakkani2016multi proposed a single recurrent neural network for modeling slot filling and intent detection jointly. liu2016attention proposed an attention-based neural network for modeling the two tasks jointly. All these models outperform the pipeline models via mutual enhancement between two tasks. However, these joint models did not model the intent information for slots explicitly and just considered their correlation between the two tasks by sharing parameters.

Recently, some joint models have explored incorporating the intent information for slot filling. goo2018slot utilize a slot-gated mechanism as a special gate function to model the relationship between the intent detection and slot filling. li2018self proposed the intent augmented gate mechanism to utilize the semantic correlation between slots and intent. Our framework is significantly different from their models including: (1) both of their approaches utilize the gate mechanism to model the relationship between intent and slots. While in our model, to directly leverage the intent information in the joint model, we feed the predicted intent information directly into slot filling with Stack-Propagation framework. (2) They apply the sentence-level intent information for each word while we adopt the token-level intent information for slot filling and further ease the error propagation. wang2018bi propose the Bi-model to consider the cross-impact between the intent and slots and achieve the state-of-the-art result. zhang-etal-2019-joint propose a hierarchical capsule neural network to model the the hierarchical relationship among word, slot, and intent in an utterance. e-etal-2019-novel introduce an SF-ID network to establish the interrelated mechanism for slot filling and intent detection tasks. Compared with their works, our model can directly incorporate the intent information for slot filling explicitly with Stack-Propagation which makes the interaction procedure more interpretable, while their model just interacts with hidden state implicitly between two tasks.

6 Conclusion

In this paper, we propose a joint model for spoken language understanding with Stack-Propagation to better incorporate the intent information for slot filling. In addition, we perform the token-level intent detection to improve the intent detection performance and further ease the error propagation. Experiments on two datasets show the effectiveness of the proposed models and achieve the state-of-the-art performance. Besides, we explore and analyze the effect of incorporating strong pre-trained BERT model in SLU tasks. With BERT, the result reaches a new state-of-the-art level.

Acknowledgments

We thank the anonymous reviewers for their helpful comments and suggestions. This work was supported by the National Natural Science Foundation of China (NSFC) via grant 61976072, 61632011 and 61772153.

References