Create PULL_REQUEST_TEMPLATE.md (#6660)

* 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>

.github/PULL_REQUEST_TEMPLATE.md

+<!-- This line specifies which issue to close after the pull request is merged. -->
+Fixes #{issue number}