Benchmark IHVP Calculator

[fel-thomas]

as discussed by @lucashervier, it would be good to have a way to properly benchmark our inverse hessian vector product calculator on different parameter dimensions -- mnist, cifar, imagenet convolution networks for example -- in order to have benchmarks on the amount of memory needed to build the inverse hessian in each case.

We still need to figure out if we integrate that in the test suite, or if we provide a notebook to benchmark ? We could meet at some point to discuss about it (@lucashervier, @Agustin-Picard). :)