Initial github issues template (#4129)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4129

Creating skeleton when creating new issues.

Instead of blank page, let's make it easy to create issues by asking users to fill out a template.

As a fallback, we still have users to submit an issue without using the template.

Reviewed By: larryliu0820

Differential Revision: D59286701

fbshipit-source-id: 022de0717db1709171ae0b8e8ae44bb72b0ab002