NeurIPS 2020

TaylorGAN: Neighbor-Augmented Policy Update Towards Sample-Efficient Natural Language Generation

Review 1

Summary and Contributions: This paper proposed a novel update formula for GAN on natural language generation (NLG) task, which is called TaylorGAN. This model overcomes the difficulty of non-differentiable operation by the first-order Taylor expansion, which can incorporate the gradient information from the discriminator. Meanwhile, this paper also used discriminator constraint and entropy maximization to achieve reliable rewards and avoid mode dropping. The experimental results illustrated the higher performance of TaylorGAN than other comparative models.

Strengths: 1. Authors firstly used Taylor expansion to approximate the discrete outputs with the non-differentiable operation, and this method is able to alleviate the high variance and low sample efficiency problems caused by the sensitivity of sampling. 2. Apart from the Taylor estimator on discrete variables (tokens in NLG), authors also proposed the discriminator constraint to improve the rewards learning. The proposed reward is unbounded and doesn’t occur in the final non-linear transformation, so it’s coordinated with the Taylor estimator. 3. The experiment is reasonable and sufficient, and the results support well the claim of the effectiveness of the proposed approach.

Weaknesses: 1. As the Taylor estimator is a keystone in the proposed model, and the motivation is to improve the estimator for discrete random variable. This paper lacks the essential analysis and comparation between Taylor estimator and traditional methods, such as the Gumbel-Softmax and Gaussian-Softmax. 2. This paper lacks the data or experiment to verify the outperformance of Taylor estimator than other simplex on discrete random variable. I suggest authors to use the synthetic data to verify whether Taylor estimator has a more continuous modality than Gumbel-Softmax and others. 3. Authors didn’t make any case study to illustrate the generated text from TaylorGAN, even though they had compared the FED and LM scored in Table.1.

Correctness: yes

Clarity: yes

Relation to Prior Work: yes

Reproducibility: Yes

Additional Feedback:

Review 2

Summary and Contributions: To solve a high variance problem in reinforcement-based text generation, the work proposes a TaylorGAN, which uses the first-order Taylor expansion to approximate the reward of generated samples around their neighbor. The methodology is supported by solid explanation and theoretical basis. The experiment shows favorable results compared with previous GAN works and the MLE model.

Strengths: 1. The idea of using the first-order Taylor expansion to approximate the reward of partial generated sentence during is novel. 2. The methodology is supported by solid theoretical proof. The experimental results as well as the generated samples also look good. 3. Without the MLE pretraining, the proposed GAN model shows comparable or even better results than previous GAN models and MLE model.

Weaknesses: 1. Experiments should cover more datasets or tasks to prove the generalization of effectiveness, such as COCO captions with less average tokens or conditional generation tasks. Perplexity metric may have small problems.

Correctness: To my best knowledge, the method is correct (though I may miss some theoretical flaws).

Clarity: The writing is easy to follow.

Relation to Prior Work: The work should discuss more related works, especially the comparison with previous GAN works if space is allowed.

Reproducibility: No

Additional Feedback: Questions: 1. In equation (11), will the importance sampling of y also cause high-variance during the estimation? Is this also affected by the accuracy of neighborhood distribution (joint distribution) in equation 13? If yes, what's the performance if you use other types of kernel functions? 2. When you evaluate the perplexity, using your own model to calculate the perplexity may be problematic since it may favor the samples generated by itself. Why don't use pre-trained language models to evaluate the PPL? e.g., GPT-2 Other comments: 1. In line 103. It is helpful to elaborate more or mention the joint distribution will be introduced in 3.1.2, otherwise, the reader will confuse how do you construct this type of joint distribution. After the rebuttal: The author's responses solved my concerns. I have two strong recommendations for the author in the revision if the paper accepted by the conference: 1. Further clarify the questions raised in the reviews. Add more comparison discussions with previous text GANs. 2. Add more datasets (at least COCO captions dataset) in the experiment part. This will make the experiment more convincing.

Review 3

Summary and Contributions: This paper proposed a new GAN model for text generation utilizing Taylor Expansion.

Strengths: The idea of the algorithm is novel

Weaknesses: The evalation is not convincing. It is only tested on one dataset and the way to show the results are problematic. I highly recommend plot quality-diversity curve given temperature sweep. See this paper's evaluation as an example :

Correctness: See weakness.

Clarity: Yes.

Relation to Prior Work: Adequite. But text generation has various different applications, different evaluation metrics are weighted differently for different tasks. It seems the authors weren't too clear on this.

Reproducibility: Yes

Additional Feedback: I would want to see more datasets in evaluation and maybe do it only for unconditional generation but also for conditional generation.

Review 4

Summary and Contributions: The paper proposes a novel update formula for the generator in a GAN-based text generation model. The model uses Tylor expansion to quickly approximate the rewards of neighboring sentences, so that the generator can learn more efficiently. TaylorGAN achieves state-of-the-art performance without maximum likelihood pre-training. The proposed model can achieve low variance without additional variance reduction techniques.

Strengths: The idea of using the gradients in the discriminator to speed up the REINFORCE learning is a novel idea. And the idea is also intuitive as shown in Figure 1. The paper clearly specifies the distribution they pick and the derives the update formula. Experimental results show that TaylorGAN achieves state-of-the-art performance.

Weaknesses: This is a well written paper, but authors can further improve the paper by giving more intuitions and overviews before writing equations. For example, the first paragraph in section 3.1 can give a brief explanation of the algorithm like “”” In our model, when we sample a sentence x and get its reward R, we can efficiently approximate neighboring sentences using first order Tylor expansion of R. Estimating more sentences can reduce variance. But we have to solve xxxx challenges. “”” Also consider refer to Figure 2 and explain the figure in this paragraph. Line 123 "we ignore the effect of v on the rest of auto-regressive sampling. " I understand it is a necessary trade-off. But in natural language, substitute one word can change the distribution of the following words a lot. Authors can consider discuss the potential effect of this approximation. The authors do not provide their implementation, which can increase the difficulty of reproduction.

Correctness: likely to be correct.

Clarity: This is a well-written paper. The related works, the method and experiments is clearly explained.

Relation to Prior Work: Yes.

Reproducibility: Yes

Additional Feedback: In Table 2, consider filling in all the omitted numbers. Line 71 samples a new token x_t ***from*** a soft policy \pi Line 133 remind reader that \Lambda appears in Eq (13). 
 In the examples shown in the appendix, the sentences generated by TaylorGAN are shorter than other methods. Is it an expected behavior? Would sentence length affect the evaluation metrics? If space allows, please add a few generated examples to the main paper.