## Automatic differentiation

Posted on 2018-06-03 | Comments

This post serves as a note and explainer of autodiff. It is licensed under GNU FDL.

For my learning I benefited a lot from Toronto CSC321 slides and the autodidact project which is a pedagogical implementation of Autograd. That said, any mistakes in this note are mine (especially since some of the knowledge is obtained from interpreting slides!), and if you do spot any I would be grateful if you can let me know.

Automatic differentiation (AD) is a way to compute derivatives. It does so by traversing through a computational graph using the chain rule.

There are the forward mode AD and reverse mode AD, which are kind of symmetric to each other and understanding one of them results in little to no difficulty in understanding the other.

In the language of neural networks, one can say that the forward mode AD is used when one wants to compute the derivatives of functions at all layers with respect to input layer weights, whereas the reverse mode AD is used to compute the derivatives of output functions with respect to weights at all layers. Therefore reverse mode AD (rmAD) is the one to use for gradient descent, which is the one we focus in this post.

Basically rmAD requires the computation to be sufficiently decomposed, so that in the computational graph, each node as a function of its parent nodes is an elementary function that the AD engine has knowledge about.

For example, the Sigmoid activation \(a' = \sigma(w a + b)\) is quite simple, but it should be decomposed to simpler computations:

- \(a' = 1 / t_1\)
- \(t_1 = 1 + t_2\)
- \(t_2 = \exp(t_3)\)
- \(t_3 = - t_4\)
- \(t_4 = t_5 + b\)
- \(t_5 = w a\)

Thus the function \(a'(a)\) is decomposed to elementary operations like addition, subtraction, multiplication, reciprocitation, exponentiation, logarithm etc. And the rmAD engine stores the Jacobian of these elementary operations.

Since in neural networks we want to find derivatives of a single loss function \(L(x; \theta)\), we can omit \(L\) when writing derivatives and denote, say \(\bar \theta_k := \partial_{\theta_k} L\).

In implementations of rmAD, one can represent the Jacobian as a transformation \(j: (x \to y) \to (y, \bar y, x) \to \bar x\). \(j\) is called the *Vector Jacobian Product* (VJP). For example, \(j(\exp)(y, \bar y, x) = y \bar y\) since given \(y = \exp(x)\),

\(\partial_x L = \partial_x y \cdot \partial_y L = \partial_x \exp(x) \cdot \partial_y L = y \bar y\)

as another example, \(j(+)(y, \bar y, x_1, x_2) = (\bar y, \bar y)\) since given \(y = x_1 + x_2\), \(\bar{x_1} = \bar{x_2} = \bar y\).

Similarly,

- \(j(/)(y, \bar y, x_1, x_2) = (\bar y / x_2, - \bar y x_1 / x_2^2)\)
- \(j(\log)(y, \bar y, x) = \bar y / x\)
- \(j((A, \beta) \mapsto A \beta)(y, \bar y, A, \beta) = (\bar y \otimes \beta, A^T \bar y)\).
- etc...

In the third one, the function is a matrix \(A\) multiplied on the right by a column vector \(\beta\), and \(\bar y \otimes \beta\) is the tensor product which is a fancy way of writing \(\bar y \beta^T\). See numpy_vjps.py for the implementation in autodidact.

So, given a node say \(y = y(x_1, x_2, ..., x_n)\), and given the value of \(y\), \(x_{1 : n}\) and \(\bar y\), rmAD computes the values of \(\bar x_{1 : n}\) by using the Jacobians.

This is the gist of rmAD. It stores the values of each node in a forward pass, and computes the derivatives of each node exactly once in a backward pass.

It is a nice exercise to derive the backpropagation in the fully connected feedforward neural networks (e.g. the one for MNIST in Neural Networks and Deep Learning) using rmAD.

AD is an approach lying between the extremes of numerical approximation (e.g. finite difference) and symbolic evaluation. It uses exact formulas (VJP) at each elementary operation like symbolic evaluation, while evaluates each VJP numerically rather than lumping all the VJPs into an unwieldy symbolic formula.

Things to look further into: the higher-order functional currying form \(j: (x \to y) \to (y, \bar y, x) \to \bar x\) begs for a functional programming implementation.