Fix docs and add docstrings for YOLOv5

[zhiqwang]

Try to fix the docs in yolort.models

[codecov]

Codecov Report

Merging #315 (b8e37dc) into main (f661460) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #315   +/-   ##
=======================================
  Coverage   94.98%   94.98%           
=======================================
  Files          11       11           
  Lines         738      738           
=======================================
  Hits          701      701           
  Misses         37       37           

Flag	Coverage Δ
unittests	94.98% <ø> (ø)

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