* [xdoctest] reformat example code with google style in `paddle/tensor/math`

* preview, test=docs_preview

* scale, test=docs_preview

* stanh, test=docs_preview

* multiplex, test=docs_preview

* pow, test=docs_preview

* max, test=docs_preview

* others..., test=docs_preview

* update trunc example code, test=docs_preview