Add pull request template

Author: Yiling-J

.github/pull_request_template.md

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+**What is this feature?**
+
+[Add a brief description of what the feature or update does.]
+
+**Why do we need this feature?**
+
+[Add a description of the problem the feature is trying to solve.]
+
+**Who is this feature for?**
+
+[Add information on what kind of user the feature is for.]
+
+**Which issue(s) does this PR fix?**:
+
+Fixes #
+
+**Special notes for your reviewer:**