Move load_from_ultralytics into _checkpoint.py

[zhiqwang]

This is a follow-up PR of #368

[codecov]

Codecov Report

Merging #373 (41dafec) into main (7f81427) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #373   +/-   ##
=======================================
  Coverage   98.58%   98.58%           
=======================================
  Files          11       11           
  Lines         778      778           
=======================================
  Hits          767      767           
  Misses         11       11           

Flag	Coverage Δ
unittests	98.58% <100.00%> (ø)

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Impacted Files	Coverage Δ
test/test_utils.py	100.00% <100.00%> (ø)

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