[Enhance]: update pr template

[ZwwWayne]

Thanks for your contribution and we appreciate it a lot. The following instructions would make your pull request more healthy and more easily get feedback. If you do not understand some items, don't worry, just make the pull request and seek help from maintainers.

Motivation

This PR updates the PR template.

Modification

The checklist in the template has been updated and use check mark for better usage.

BC-breaking (Optional)

N/A

Use cases (Optional)

N/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.

[codecov]

Codecov Report

Merging #1228 (b844bf5) into master (571e3e5) will increase coverage by 0.15%.
The diff coverage is 72.72%.

❗ Current head b844bf5 differs from pull request most recent head bb4b3e7. Consider uploading reports for the commit bb4b3e7 to get more accurate results

@@            Coverage Diff             @@
##           master    #1228      +/-   ##
==========================================
+ Coverage   68.27%   68.42%   +0.15%     
==========================================
  Files         160      160              
  Lines       10599    10610      +11     
  Branches     1937     1938       +1     
==========================================
+ Hits         7236     7260      +24     
+ Misses       2979     2975       -4     
+ Partials      384      375       -9     

Flag	Coverage Δ
unittests	68.42% <72.72%> (+0.15%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmcv/__init__.py	100.00% <ø> (ø)
mmcv/arraymisc/__init__.py	100.00% <ø> (ø)
mmcv/arraymisc/quantization.py	100.00% <ø> (ø)
mmcv/cnn/__init__.py	100.00% <ø> (ø)
mmcv/cnn/alexnet.py	26.08% <ø> (ø)
mmcv/cnn/bricks/__init__.py	100.00% <ø> (ø)
mmcv/cnn/bricks/activation.py	87.50% <ø> (ø)
mmcv/cnn/bricks/context_block.py	96.96% <ø> (ø)
mmcv/cnn/bricks/conv.py	100.00% <ø> (ø)
mmcv/cnn/bricks/conv2d_adaptive_padding.py	94.73% <ø> (ø)
... and 93 more

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[zhouzaida]

LGTM