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Title: WiSE-ALE: Wide Sample Estimator for Aggregate Latent Embedding

Paper content:
In this paper, we present a new generative model for learning latent embeddings.
Compared to the classical generative process, where each observed data point is generated from an individual latent variable, our approach assumes a global latent variable to generate the whole set of observed data points.
We then propose a learning objective that is derived as an approximation to a lower bound to the data log likelihood, leading to our algorithm, WiSE-ALE.
Compared to the standard ELBO objective, where the variational posterior for each data point is encouraged to match the prior distribution, the WiSE-ALE objective matches the averaged posterior, over all samples, with the prior, allowing the sample-wise posterior distributions to have a wider range of acceptable embedding mean and variance and leading to better reconstruction quality in the auto-encoding process.
Through various examples and comparison to other state-of-the-art VAE models, we demonstrate that WiSE-ALE has excellent information embedding properties, whilst still retaining the ability to learn a smooth, compact representation.
Unsupervised learning is a central task in machine learning.
Its objective can be informally described as learning a representation of some observed forms of information in a way that the representation summarizes the overall statistical regularities of the data BID0 .
Deep generative models are a popular choice for unsupervised learning, as they marry deep learning with probabilistic models to estimate a joint probability between high dimensional input variables x and unobserved latent variables z.
Early successes of deep generative models came from Restricted Boltzmann Machines BID7 and Deep Boltzmann Machines BID15 , which aim to learn a compact representation of data.
However, the fully stochastic nature of the network requires layer-by-layer pre-training using MCMC-based sampling algorithms, resulting in heavy computation cost.
BID9 consider the objective of optimizing the parameters in an auto-encoder network by deriving an analytic solution to a variational lower bound of the log likelihood of the data, leading to the Auto-Encoding Variational Bayes (AEVB) algorithm.
They apply a reparameterization trick to maximally utilize deterministic mappings in the network, significantly simplifying the training procedure and reducing instability.
Furthermore, a regularization term naturally occurs in their model, allowing a prior p(z) to be placed over every sample embedding q(z|x).
As a result, the learned representation becomes compact and smooth; see e.g. FIG0 where we learn a 2D embedding of MNIST digits using 4 different methods and visualize the aggregate posterior distribution of 64 random samples in the learnt 2D embedding space.
However, because the choice of the prior is often uninformative, the smoothness constraint imposed by this regularization term can cause information loss between the input samples and the latent embeddings, as shown by the merging of individual embedding distributions in FIG0
(d) (especially in the outer areas away from zero code).
Extreme effects of such behaviours can be noticed from β-VAE BID6 , a derivative algorithm of AEVB which further increases the weighting on the regularizing term with the aim of learning an even smoother, disentangled representation of the data.
As shown in FIG0
(e), the individual embedding distributions are almost indistinguishable, leading to an overly severe information bottleneck which can cause high rates of distortion BID16 .
In contrast, perfect reconstruction can be achieved using WAE (Tolstikhin et al., 2017) , but the learnt embedding distributions appear to severely non-smooth ( FIG0 ), indicating a small amount of noise in the latent space would cause generation process to fail.In this paper, we propose WiSE-ALE (a wide sample estimator), which imposes a prior on the bulk statistics of a mini-batch of latent embeddings.
Learning under our WiSE-ALE objective does not penalize individual embeddings lying away from the zero code, so long as the aggregate distribution (the average of all individual embedding distributions) does not violate the prior significantly.
Hence, our approach mitigates the distortion caused by the current form of the prior constraint in the AEVB objective.
Furthermore, the objective of our WiSE-ALE algorithm is derived by applying variational inference in a simple latent variable model (Section
2) and with further approximation, we derive an analytic form of the learning objective, resulting in efficient learning algorithm.In general, the latent representation learned using our algorithm enjoys the following properties:
1) smoothness, as indicated in FIG0 , the probability density for each individual embedding distribution decays smoothly from the peak value;
2) compactness, as individual embeddings tend to occupy a maximal local area in the latent space with minimal gaps in between; and
3) separation, indicated by the narrow, but clear borders between neighbouring embedding distributions as opposed to the merging seen in AEVB.
In summary, our contributions are:• proposing a new latent variable model that uses a single global latent variable to generate the entire dataset,• deriving a variational lower bound to the data log likelihood in our latent variable model, which allows us to impose prior constraint on the bulk statistics of a mini-batch embedding distributions,• and deriving analytic approximations to the lower bound, leading to our efficient WiSE-ALE learning algorithm.In the rest of the paper, we first review directed graphical models in Section
2. We then derive our variational lower bound and its analytic approximations in Section
3. Related work is discussed in Section
4. Experiment results are analyzed in Section 5, leading to conclusions in Section 6.
In this paper, we propose a new latent variable model where a global latent variable is used to generate the entire dataset.
We then derive a variational lower bound to the data log likelihood, which allows us to impose a prior constraint on the bulk statistics of the aggregate posterior distribution for the entire dataset.
Using an analytic approximation to this lower bound as our learning objective, we propose WiSE-ALE algorithm.
We have demonstrated its ability to achieve excellent reconstruction quality, as well as forming a smooth, compact and meaningful latent representation.
In the future, we would like to understand the properties of the latent embeddings learnt through our method and apply it for suitable applications.
In this appendix, we omit the trainable parameters φ and θ in the expressions of distributions for simplicity.
For example, q(z|x) is equivalent to q φ (z|x) and p(x|z) represents p θ (x|z).

assistant: text: We propose a new latent variable model to learn latent embeddings for some high-dimensional data.