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.

首先，将 Python 文档中说明的 reduce_mean、reduce_std 和 reduce_variance 操作与现有的 math.mean 实现以及 issue 中引用的 API 定义 proto 进行比较。查看 issue #117 了解相关背景；完成意味着预期的操作及其 proto 或 wrapper 要求已得到解决，并由 Java API 覆盖。