Discretized logistic distribution layer

[DavideScassola]

I have implemented a layer for the discretized logistic distribution, a distribution defined over the integers:

$$\text{logpdf}(x) = \log( \text{cdf}(x + 0.5) - \text{cdf}(x-0.5))$$ where $$\text{cdf}(x) = \frac{1}{1 + e^{-(x-\mu)/\sigma }}$$