August 31, 2016

Onur Kuru, M.S. 2016

M.S. Thesis: Character-level Tagging. Koç University, Department of Computer Engineering. August, 2016. (PDF, Presentation, Code)


I describe and evaluate a language-independent character-level tagger for sequence labeling problems: Named Entity Recognition (NER), Part-of-Speech (POS) tagging and Chunking. Instead of words, a sentence is represented as a sequence of characters. The model consists of stacked bidirectional LSTMs which input characters and output tag probabilities for each character. These probabilities are then converted to consistent word level phrase tags using a Viterbi decoder. The model uses only labeled data and does not rely on hand-engineered features or other external resources like syntactic taggers or Gazetteers. The model is able to achieve close to state-of-the-art NER performance in seven languages, performs as well as or better than previous work in four languages for POS tagging and yields competitive results for English Chunking dataset.

Full post...

August 23, 2016


AutoGrad.jl is an automatic differentiation package for Julia. It is a Julia port of the popular Python autograd package. It can differentiate regular Julia code that includes loops, conditionals, helper functions, closures etc. by keeping track of the primitive operations and using this execution trace to compute gradients. It uses reverse mode differentiation (a.k.a. backpropagation) so it can efficiently handle functions with array inputs and scalar outputs. It can compute gradients of gradients to handle higher order derivatives. Please see the comments in core.jl for a description of how the code works in detail.


You can install AutoGrad in Julia using:

julia> Pkg.add("AutoGrad")

In order to use it in your code start with:

using AutoGrad

Here is a linear regression example simplified from housing.jl:

using AutoGrad

function loss(w)
    global xtrn,ytrn
    ypred = w[1]*xtrn .+ w[2]
    sum(abs2(ypred - ytrn)) / size(ypred,2)

function train(w; lr=.1, epochs=20)
    gradfun = grad(loss)
    for epoch=1:epochs
        g = gradfun(w)
        for i in 1:length(w)
            w[i] -= lr * g[i]
    return w

The loss function takes parameters as input and returns the loss to be minimized. The parameter w for this example is a pair: w[1] is a weight matrix, and w[2] is a bias vector. The training data xtrn,ytrn are in global variables. ypred is the predicted output, and the last line computes the quadratic loss. The loss function is implemented in regular Julia.

The train function takes initial parameters and returns optimized parameters. grad is the only AutoGrad function used: it creates a function gradfun that takes the same arguments as loss, but returns the gradient instead. The returned gradient will have the same type and shape as the input argument. The for loop implements gradient descent, where we calculate the gradient and subtract a scaled version of it from the weights.

See the examples directory for more examples, and the extensively documented core.jl for details.

Extending AutoGrad

AutoGrad can only handle a function if the primitives it uses have known gradients. You can add your own primitives with gradients as described in detail in core.jl or using the @primitive and @zerograd macros in util.jl Here is an example:

@primitive hypot(x1::Number,x2::Number)::y  (dy->dy*x1/y)  (dy->dy*x2/y)

The @primitive macro marks the hypot(::Number,::Number) method as a new primitive and the next two expressions define gradient functions wrt the first and second argument. The gradient expressions can refer to the parameters and the return variable (indicated after the final ::) of the method declaration.

Note that Julia supports multiple-dispatch, i.e. a function may have multiple methods each supporting different argument types. For example hypot(x1::Array,x2::Array) is another hypot method. In AutoGrad.jl each method can independently be defined as a primitive and can have its own specific gradient.

Code structure

core.jl implements the main functionality and acts as the main documentation source. util.jl has some support functions to define and test new primitives. interfaces.jl sets up support for common data structures including Arrays, Tuples, and Dictionaries. The numerical gradients are defined in files such as base/math.jl, special/trig.jl that mirror the organization under julia/base.

Current status and future work

The gradient coverage is spotty, I am still adding more gradients to cover the Julia base. Next steps are to make models faster by providing support for GPU operations and overwriting functions (to avoid memory allocation). I should also find out about the efficiency of closures and untyped functions in Julia which are used extensively in the code.

Acknowledgments and references

AutoGrad.jl was written by Deniz Yuret. Large parts of the code are directly ported from the Python autograd package. I'd like to thank autograd author Dougal Maclaurin for his support. See (Baydin et al. 2015) for a general review of automatic differentiation, autograd tutorial for some Python examples, and Dougal's PhD thesis for design principles. JuliaDiff has alternative differentiation tools for Julia. I would like to thank my students Ozan Arkan Can and Emre Yolcu for helpful contributions.

Also see: A presentation, A demo.

Full post...