[chore] Updating PR template (#674)

* [chore] Updating PR template

Add N/A (Not Applicable) options to some of the questions in the PR template

* Update PULL_REQUEST_TEMPLATE.md

* Update PULL_REQUEST_TEMPLATE.md

.github/PULL_REQUEST_TEMPLATE.md

-# Before submitting
+## What does this PR do?
+Fixes # (issue).
 
-- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
+## Before submitting
+
+- [ ] Did you have fun?
+  - Make sure you had fun coding 🙃
 - [ ] Did you read the [contributor guideline](https://github.com/facebookresearch/fairscale/blob/master/CONTRIBUTING.md)?
+- [ ] Was this discussed/approved via a Github issue? (no need for typos, doc improvements)
+  - [ ] N/A
 - [ ] Did you make sure to update the docs?
+  - [ ] N/A
 - [ ] Did you write any new necessary tests?
+  - [ ] N/A
 - [ ] Did you update the [changelog](https://github.com/facebookresearch/fairscale/blob/master/CHANGELOG.md)? (if needed)
+  - [ ] N/A
 
 
-## What does this PR do?
-Fixes # (issue).
-
 ## PR review
 Anyone in the community is free to review the PR once the tests have passed.
 If we didn't discuss your PR in Github issues there's a high chance it will not be merged.
-
-## Did you have fun?
-Make sure you had fun coding 🙃