Update bambu pr template

[lingminhao]

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (please specify if the change breaks existing functionality)
  • Non breaking change (the feature doesn't change existing functionality)
  • Breaking change (the feature that would cause existing functionality to not work as expected)
• Documentation update
• Performance optimization

Description

addressed all the issues mentioned about pull request template during bambu hackathon. In particular,

• add to checklist if all tests pass (should be done before reviewers are invited)
• add to checklist: no style changes in unaffected code
• add a section below the checklist to list downstream issues/known issues that reviewers do not need to comment on, e.g. tests will not pass as they did not pass in the original branch, this will be fixed later; unit tests will fail as part of the code changes for functions a(), b(), etc; unit tests will be updated after code review; list of remaining bugs that are known and fill be fixed later as part of another issue (please add a #TODO comment in the affected code)
• add note to remove unused sections and the reference text. PR template is only template, only useful parts should be kept
• description and user input are not separated in terms of font size and font type, hard to see

Impact of Changes

• only the template for pull request has changed