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.

まず、Pythonでドキュメント化されている reduce_mean、reduce_std、reduce_variance の各操作を、既存の math.mean 実装および issue で参照されている API 定義 proto と比較します。関連するコンテキストについては issue #117 を確認してください。完了とは、意図された操作と、それらに必要な proto または wrapper の要件が解決され、Java API でカバーされていることを意味します。