# NormalizingFlows.jl

Documentation for NormalizingFlows.

The purpose of this package is to provide a simple and flexible interface for variational inference (VI) and normalizing flows (NF) for Bayesian computation and generative modeling. The key focus is to ensure modularity and extensibility, so that users can easily construct (e.g., define customized flow layers) and combine various components (e.g., choose different VI objectives or gradient estimates) for variational approximation of general target distributions, without being tied to specific probabilistic programming frameworks or applications.

See the documentation for more.

## Installation

To install the package, run the following command in the Julia REPL:

]  # enter Pkg mode
(@v1.9) pkg> add git@github.com:TuringLang/NormalizingFlows.jl.git

Then simply run the following command to use the package:

using NormalizingFlows

## What are normalizing flows?

Normalizing flows transform a simple reference distribution $q_0$ (sometimes known as base distribution) to a complex distribution $q_\theta$ using invertible functions with trainable parameter $\theta$, aiming to approximate a target distribution $p$. The approximation is achieved by minimizing some statistical distances between $q$ and $p$.

In more details, given the base distribution, usually a standard Gaussian distribution, i.e., $q_0 = \mathcal{N}(0, I)$, we apply a series of parameterized invertible transformations (called flow layers), $T_{1, \theta_1}, \cdots, T_{N, \theta_k}$, yielding that

$$$Z_N = T_{N, \theta_N} \circ \cdots \circ T_{1, \theta_1} (Z_0) , \quad Z_0 \sim q_0,\quad Z_N \sim q_{\theta},$$$

where $\theta = (\theta_1, \dots, \theta_N)$ are the parameters to be learned, and $q_{\theta}$ is the transformed distribution (typically called the variational distribution or the flow distribution). This describes sampling procedure of normalizing flows, which requires sending draws from the base distribution through a forward pass of these flow layers.

Since all the transformations are invertible (technically diffeomorphic), we can evaluate the density of a normalizing flow distribution $q_{\theta}$ by the change of variable formula:

$$$q_\theta(x)=\frac{q_0\left(T_1^{-1} \circ \cdots \circ T_N^{-1}(x)\right)}{\prod_{n=1}^N J_n\left(T_n^{-1} \circ \cdots \circ T_N^{-1}(x)\right)} \quad J_n(x)=\left|\operatorname{det} \nabla_x T_n(x)\right|.$$$

Here we drop the subscript $\theta_n, n = 1, \dots, N$ for simplicity. Density evaluation of normalizing flow requires computing the inverse and the Jacobian determinant of each flow layer.

Given the feasibility of i.i.d. sampling and density evaluation, normalizing flows can be trained by minimizing some statistical distances to the target distribution $p$. The typical choice of the statistical distance is the forward and reverse Kullback-Leibler (KL) divergence, which leads to the following optimization problems:

\begin{aligned} \text{Reverse KL:}\quad &\argmin _{\theta} \mathbb{E}_{q_{\theta}}\left[\log q_{\theta}(Z)-\log p(Z)\right] \\ &= \argmin _{\theta} \mathbb{E}_{q_0}\left[\log \frac{q_\theta(T_N\circ \cdots \circ T_1(Z_0))}{p(T_N\circ \cdots \circ T_1(Z_0))}\right] \\ &= \argmax _{\theta} \mathbb{E}_{q_0}\left[ \log p\left(T_N \circ \cdots \circ T_1(Z_0)\right)-\log q_0(X)+\sum_{n=1}^N \log J_n\left(F_n \circ \cdots \circ F_1(X)\right)\right] \end{aligned}

and

\begin{aligned} \text{Forward KL:}\quad &\argmin _{\theta} \mathbb{E}_{p}\left[\log q_{\theta}(Z)-\log p(Z)\right] \\ &= \argmin _{\theta} \mathbb{E}_{p}\left[\log q_\theta(Z)\right] \end{aligned}

Both problems can be solved via standard stochastic optimization algorithms, such as stochastic gradient descent (SGD) and its variants.