Add pre-commit hooks

[zhiqwang]

• add flake8 as format checker, refer to add pre-commit hooks for convenient formatting checks pytorch/vision#4387
• add ufmt as code formatter, refer to add ufmt as code formatter pytorch/vision#4384
• add mdformat as markdown formatter

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[codecov]

Codecov Report

Merging #182 (07a72f3) into master (97f507d) will increase coverage by 0.31%.
The diff coverage is 98.07%.

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   96.17%   96.48%   +0.31%     
==========================================
  Files          10       10              
  Lines         601      598       -3     
==========================================
- Hits          578      577       -1     
+ Misses         23       21       -2     

Flag	Coverage Δ
unittests	96.48% <98.07%> (+0.31%)	⬆️

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

Impacted Files	Coverage Δ
test/test_relaying.py	100.00% <ø> (ø)
test/test_v5.py	93.33% <75.00%> (+5.83%)	⬆️
test/test_utils.py	98.38% <94.44%> (+1.56%)	⬆️
test/test_data_pipeline.py	100.00% <100.00%> (ø)
test/test_engine.py	100.00% <100.00%> (ø)
test/test_models.py	95.37% <100.00%> (-0.03%)	⬇️
test/test_models_anchor_utils.py	100.00% <100.00%> (ø)
test/test_models_common.py	100.00% <100.00%> (ø)
test/test_models_transform.py	100.00% <100.00%> (ø)
test/test_onnx.py	85.24% <100.00%> (ø)

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