为 MultivariateNormal 分布添加中文文档

[dasenCoding]

为MultivariateNormal 分布，添加中文API文档。

[paddle-bot[bot]]

感谢你贡献飞桨文档，文档预览构建中，Docs-New 跑完后即可预览，预览链接：http://preview-pr-5551.paddle-docs-preview.paddlepaddle.org.cn/documentation/docs/zh/api/index_cn.html 预览工具的更多说明，请参考：飞桨文档预览工具

[CLAassistant]

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