Lipschitz constant estimator

[thib-s]

The function evaluate_lip_const in the deel.lip.utils module use a sampled method. Although this is acceptable in a low dimensional space (as done in tests) the measured constant can be heavily underestimated in high dimensional space.

There is a need of building a module to evaluate accurately the lipschitz constant of a layer. Especially for Dense and Conv layers.

[cofri]

Do we consider this issue solved with PR #81?