Possibly missing op functions: reduce mean/std/variance, std, variance?

I've noticed that a few ops present in the python docs are missing for reasons I can't determine. Specifically reduce_mean, reduce_std, and reduce_variance. reduce_mean seems like it is just missing the reduce wrapper (it's present in math.mean) but I can't find methods for standard deviation or variance at all. I thought it might be related to #117 so I went looking through the api def protos, and there aren't any standard deviation or variance protos, but there aren't any reduce op protos either so I'm not sure what's up. Regardless of whether this is #117 or not, variance at least would be nice to have.

Beginne damit, die in Python dokumentierten Operationen reduce_mean, reduce_std und reduce_variance mit der vorhandenen Implementierung math.mean und den in der Issue referenzierten API-Definitions-Protos zu vergleichen. Sieh dir Issue #117 wegen des zugehörigen Kontexts an; abgeschlossen bedeutet, dass die vorgesehenen Operationen und ihre Proto- oder Wrapper-Anforderungen geklärt und durch die Java API abgedeckt sind.