BIGAUD Nathan authored 2 years ago and ANDREY Paul committed 2 years ago

* Implement `GaussianNoiseModule`, to enable adding gaussian noise
  to step-wise batch-averaged gradients as part of local DP-SGD or
  to clients-averaged updates as part of central DP-SGD.
* Implement an abstract `NoiseModule` layer, paving the way for
  other noise-addition mechanisms.

This commit squashes some incremental work by multiple authors:
* Nathan Bigaud implemented `GaussianNoiseModule` after doing the
  scientific and technical review of requirements, notably as to
  the use of a cryptographically-safe RNG.
* Paul Andrey abstracted part of the code into the `NoiseModule`
  class, fixed some pending todos, and adjusted unit tests.

Co-authored-by: Paul Andrey <paul.andrey@inria.fr>