A simple and clever (but inefficient) way to calculate M-estimator sensitivity with automatic differentiation.

sensitivity
Published

August 29, 2019

I have some recent work (A Higher-Order Swiss Army Infinitesimal Jackknife) that is all about calculating Taylor expansions of optima with respect to hyperparameters using automatic differentiation.

In the paper we talk about sensitivity to data weights, but the idea is much more general. Suppose you have a parameter which you’re optimizing, $$\theta$$, and some hyperparameter $$\epsilon$$. For a fixed $$\epsilon$$, you find $$\hat\theta(\epsilon)$$ to satisfy some vector of first-order conditions,

$G(\hat\theta(\epsilon), \epsilon) = 0.$

For example, if you’re optimizing $$F(\theta, \epsilon)$$ for a fixed $$\epsilon$$, $$G$$ would be the vector of partial derivatives of $$F$$ with respect to $$\theta$$.

Of course, $$\hat\theta(\epsilon)$$ depends on $$\epsilon$$, and you might want to approximately calculate $$\hat\theta(\epsilon)$$ for different $$\epsilon$$ without re-solving an optimiztion problem. Specifically, you might form a Taylor series expansion around some $$\epsilon_0$$:

$\hat\theta(\epsilon) \approx \hat\theta(\epsilon_0) + \left.\frac{d \hat\theta(\epsilon)}{d\epsilon}\right|_{\epsilon_0} (\epsilon - \epsilon_0) + \frac{1}{2} \left.\frac{d^2 \hat\theta(\epsilon)}{d\epsilon^2}\right|_{\epsilon_0} (\epsilon - \epsilon_0)(\epsilon - \epsilon_0) + ...$

The difficulty is how to calculate the derivatives, which are defined implicitly through the solution of $$G(\hat\theta(\epsilon), \epsilon) = 0$$. The computation section of our paper describes one way to do so recursively, and I’ve implemented the solution in the ParametricSensitivityTaylorExpansion class of my Python package vittles.

My perspective works (and is amenable to theory), but is a bit complicated to implement. Martin Jankowiak at Uber described to me his idea for an extremely elegant, though unfortunately inefficient, implementation. Let me demonstrate his idea in autograd and discuss how it is inefficient.

First, we’ll need to use the fact that the first derivative is given by

$.|_{0} = -. |{0} ^{-1} . |{_0} ^{-1}.$

(Recall that $$G(\theta, \epsilon)$$ is a vector of the same length as $$\theta$$.) Now, suppose we have implemented $$G(\theta, \epsilon)$$ in Python, found a solution theta0 at epsilon0:

def g(theta, epsilon):
... your estimating equation here ...

g(theta0, epsilon0) # ...is a vector of zeros.

Using this, we can implement the optimal $$\hat\theta(\epsilon)$$ as the following function, which only evaluates at $$\epsilon_0$$:

def check_epsilon(epsilon):
assert np.linalg.norm(epsilon - hyperpar0) < 1e-8

@primitive
def get_thetahat(epsilon):
check_epsilon(epsilon)
return theta0

As-is, this is a useless function. It only returns what we already know, which is that theta0 is the optimum for epsilon0, and otherwise throws an error. However, we have marked it @primitive, which means we can specify a custom derivative using the formula above. We will only be able to evaluate this derivative at epsilon0, of course, but that’s all we want.

dg_dtheta = autograd.jacobian(g, argnum=0)

# Reverse mode AD is a "vector jacobian product", or "vjp".
def get_thetahat_vjp(ans, epsilon):
def vjp(g):
thetahat = get_thetahat(epsilon)
return -1 * (dg_depsilon(thetahat, epsilon).T @
np.linalg.solve(dg_dtheta(thetahat, epsilon), g)).T
return vjp

# Tell autograd to use get_thetahat_vjp to reverse-mode autodiff get_thetahat.
defvjp(get_thetahat, get_thetahat_vjp)

The fucnction vjp is simply the reverse-mode implementation of the formula above for the first derivative.

Now, the magic is that this implemenatation of the derivative of get_thetahat itself is composed of differentiable functions: linear algebra (solve and matrix multiplication), derivatives of g, and … get_thetahat itself, whose derivative we have just defined! Consequently, this single definition suffices for (reverse mode) automatic differentiation of get_thetahat of all orders. A forward mode implementation is obviously similar.

So how does it work? It works, but unfortunately, it’s quite slow for higher order derivatives, at least relative to vittles. I believe that our paper actually makes the reasons for our speedup clear.

• You’ll need to repeatedly solve systems involving $$\left.\frac{\partial G(\theta, \epsilon)}{\partial \theta^T} \right|_{\epsilon_0} ^{-1}$$ , i.e., calculate np.linalg.solve(dg_dtheta(thetahat, epsilon), ...). If you are implementing the derivatives in closed form (as I do in vittles) you can Cholesky factorize this matrix once, but autograd doesn’t know that — and can’t, because it needs to differentate through the matrix evaluation for this trick to work.
• Lower-order derivatives of $$\hat\theta(\epsilon)$$ appear in the expressions for higher-order derivatives. Again, if you are implementing the derivatives in closed form you can avoid re-caluculating these lower-order derivatives, but autodiff has no way to be aware of this redundant structure.
• Because every higher order derivative multiplies only terms of the form $$\epsilon - \epsilon_0$$, there is a lot of redundancy due to symmetry. Closed-form implementations can recognize this symmetry and avoid redundant calculations but, again, automatic differentiation is not aware of this structure.

Although vittles overcomes these difficulties, it does so at the cost of considerable complexity — as you might expect, since essentially the benefits come from caching, which is always complicated.

For more details, you can see the notebook below, which is also available here for download.

{% include_relative html/_2019-08-29-calculating_m_estimator_sensitivity.html %}