Implement NeuralStatistician

[masenka31]

Adding a new generative model working on sets of data. Work in progress now.

[nmheim]

Good stuff, thanks a lot for the PR! I requested some minor changes in the code :)

[nmheim]

Great, we are almost there now! In order to create a new release you just need to increase the version number of GenerativeModels in the Project.toml (from 0.2.2 to 0.2.3). then the julia tagbot will automatically create a new release an register it in the julia registry, so that we can install/update the package via ]add Generativemodels/]update :)

[nmheim]

This could be misunderstood. It looks like the elbo would only get a NeuralStatistician type instead of an actual object. maybe something like this:

model = NeuralStatistician(cdim, instance_enc, enc_c_dist, cond_z_dist, enc_z_dist, dec_dist)
loss(x) = - elbo(model,x)

In that case it would also be really great to have short explanations of the variables in there like instance_enc. but if you are pressed with time we can just merge it with the corrections above.

[codecov-io]

Codecov Report

Merging #96 (3e63c15) into master (f463742) will decrease coverage by 7.24%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master      #96      +/-   ##
==========================================
- Coverage   81.81%   74.57%   -7.25%     
==========================================
  Files           5        6       +1     
  Lines          88      118      +30     
==========================================
+ Hits           72       88      +16     
- Misses         16       30      +14     

Impacted Files	Coverage Δ
src/GenerativeModels.jl	100.00% <ø> (ø)
src/statistician.jl	53.33% <53.33%> (ø)

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[masenka31]

I updated the documentation for NeuralStatistician to include an example and rewrote the docs for elbo to be more accurate. I think there should be no more problems.

[nmheim]

Looks great, thank you! :)