update function doc to rst format for auto-gen (#823)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/823

Update the documentation for function into rst format to make the documentation automatic generated from doc string in our codebase.

Reviewed By: tarun292

Differential Revision: D50154620

fbshipit-source-id: ef6853134c8cced25b110812e267ae6e47c0cb7c