[Refactor] code quality

[RangiLyu]

In last few weeks, I was working on improving the code quality:

1. Use black code format to standardize code style.
2. Add unit tests:
   • test configs
   • test models
     • test modules
     • test backbones
     • test fpns
     • test heads
     • test losses
   • test datasets
   • test augs
   • test trainer
   • test evaluator
3. Improve code coverage to 73%.
4. Some refactoring.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (main@be08279). Click here to learn what that means.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #289   +/-   ##
=======================================
  Coverage        ?   73.46%           
=======================================
  Files           ?       60           
  Lines           ?     3931           
  Branches        ?      645           
=======================================
  Hits            ?     2888           
  Misses          ?      852           
  Partials        ?      191           

Flag	Coverage Δ
unittests	73.46% <0.00%> (?)

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