Implement Flipout sampling?

[dsteinberg]

https://arxiv.org/abs/1803.04386

Eqn. (4) could be implemented as an alternative to _sample_W in layers.py, caveats:

• This maybe be hard getting to work with full-covariance weights
• This may require the mean and std. dev. of the weight samples to be output independently for the vectorized implementation.