adding PR template (#4586)

* adding PR template

* update

* Update PULL_REQUEST_TEMPLATE.md

* Update PULL_REQUEST_TEMPLATE.md

* Update .github/PULL_REQUEST_TEMPLATE.md
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>
Co-authored-by: Nicolas Hug <contact@nicolas-hug.com>

.github/PULL_REQUEST_TEMPLATE.md

+<!-- Before submitting a PR, please make sure to check our contributing guidelines regarding code formatting, tests, and documentation: https://github.com/pytorch/vision/blob/main/CONTRIBUTING.md -->