Fix support for YOLOv5 nano models

[zhiqwang]

This is a follow-up PR of #199 to support YOLOv5 nano model as described in ultralytics/yolov5#5141.

[codecov]

Codecov Report

Merging #204 (fb3e81b) into main (6d08630) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #204   +/-   ##
=======================================
  Coverage   96.98%   96.98%           
=======================================
  Files          10       10           
  Lines         630      630           
=======================================
  Hits          611      611           
  Misses         19       19           

Flag	Coverage Δ
unittests	96.98% <ø> (ø)

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