[Docs] Add PR template and modify issue template

[mzr1996]

Motivation

Use PR template to standardized PR docs.
And add language suggestions in the Chinese issue templates to recommend users to use the English templates.

Modification

1. Add PR template, refers to MMCV PR template.
2. Add language suggestions in the Chinese issue templates.
3. Add issue template config, refers to MMCV issue config.

Checklist

• Pre-commit or other linting tools are used to fix the potential lint issues.
• The modification is covered by complete unit tests. If not, please add more unit tests to ensure the correctness.
• If the modification has potential influence on downstream projects, this PR should be tested with downstream projects, like MMDet or MMSeg.
• The documentation has been modified accordingly, like docstring or example tutorials.

[codecov]

Codecov Report

Merging #380 (733f8ec) into master (c92eefb) will increase coverage by 0.48%.
The diff coverage is 98.19%.

❗ Current head 733f8ec differs from pull request most recent head c995050. Consider uploading reports for the commit c995050 to get more accurate results

@@            Coverage Diff             @@
##           master     #380      +/-   ##
==========================================
+ Coverage   76.02%   76.51%   +0.48%     
==========================================
  Files          96       97       +1     
  Lines        5114     5224     +110     
  Branches      849      856       +7     
==========================================
+ Hits         3888     3997     +109     
  Misses       1108     1108              
- Partials      118      119       +1     

Flag	Coverage Δ
unittests	76.51% <98.19%> (+0.48%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/datasets/imagenet.py	24.48% <0.00%> (ø)
mmcls/datasets/pipelines/auto_augment.py	78.78% <ø> (ø)
mmcls/datasets/pipelines/transforms.py	86.81% <ø> (ø)
mmcls/models/backbones/tnt.py	99.08% <99.08%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)

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[Ezra-Yu]

OK.