[Enhance]: update pr template (#1228)

* update pr template

* reformat

* update according to comments

* resolve comments

.github/pull_request_template.md

@@ -10,7 +10,7 @@ Please briefly describe what modification is made in this PR.
 
 ## BC-breaking (Optional)
 
-Does the modification introduce changes that break the backward-compatibility of the downstream repos?
+Does the modification introduce changes that break the backward-compatibility of the downstream repositories?
 If so, please describe how it breaks the compatibility and how the downstream projects should modify their code to keep compatibility with this PR.
 
 ## Use cases (Optional)
@@ -19,7 +19,15 @@ If this PR introduces a new feature, it is better to list some use cases here, a
 
 ## Checklist
 
-1. Pre-commit or other linting tools are used to fix the potential lint issues.
-2. The modification is covered by complete unit tests. If not, please add more unit test to ensure the correctness.
-3. If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMCls.
-4. The documentation has been modified accordingly, like docstring or example tutorials.
+**Before PR**:
+
+- [ ] I have read and followed the workflow indicated in the [CONTRIBUTING.md](https://github.com/open-mmlab/mmcv/blob/master/CONTRIBUTING.md) to create this PR.
+- [ ] Pre-commit or linting tools indicated in [CONTRIBUTING.md](https://github.com/open-mmlab/mmcv/blob/master/CONTRIBUTING.md) are used to fix the potential lint issues.
+- [ ] Bug fixes are covered by unit tests, the case that causes the bug should be added in the unit tests.
+- [ ] New functionalities are covered by complete unit tests. If not, please add more unit test to ensure the correctness.
+- [ ] The documentation has been modified accordingly, including docstring or example tutorials.
+
+**After PR**:
+
+- [ ] If the modification has potential influence on downstream or other related projects, this PR should be tested with some of those projects, like MMDet or MMCls.
+- [ ] CLA has been signed and all committers have signed the CLA in this PR.