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.

Inizia confrontando le operazioni reduce_mean, reduce_std e reduce_variance documentate in Python con l’implementazione esistente di math.mean e i proto di definizione dell’API a cui si fa riferimento nella issue. Controlla la issue #117 per il contesto correlato; il lavoro è completato quando le operazioni previste e i relativi requisiti di proto o wrapper sono risolti e coperti dalla Java API.