* Create PULL_REQUEST_TEMPLATE.md
Proposing to copy this neat feature from pytorch. This is a small template that let's a PR submitter tell which issue that PR closes.
* Update .github/PULL_REQUEST_TEMPLATE.md
Co-authored-by: Kevin Canwen Xu <canwenxu@126.com>
Co-authored-by: Kevin Canwen Xu <canwenxu@126.com>