Move convert_yolov5_checkpoint into _checkpoint.py

[zhiqwang]

And rename tools/yolov5_to_yolort.py to tools/convert_yolov5_to_yolort.py.

[codecov]

Codecov Report

Merging #374 (6dabec5) into main (5942ce8) will not change coverage.
The diff coverage is n/a.

❗ Current head 6dabec5 differs from pull request most recent head b62a1bf. Consider uploading reports for the commit b62a1bf to get more accurate results

@@           Coverage Diff           @@
##             main     #374   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files          11       11           
  Lines         778      778           
=======================================
  Hits          767      767           
  Misses         11       11           

Flag	Coverage Δ
unittests	98.58% <ø> (ø)

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