Move Lightning wrapper into trainer

[zhiqwang]

Make the YOLOv5 Module clean, move the Lightning wrapper into yolort.trainer.

[codecov]

Codecov Report

Merging #314 (58bfb65) into main (a165e17) will increase coverage by 0.04%.
The diff coverage is 100.00%.

❗ Current head 58bfb65 differs from pull request most recent head 8ebef4c. Consider uploading reports for the commit 8ebef4c to get more accurate results

@@            Coverage Diff             @@
##             main     #314      +/-   ##
==========================================
+ Coverage   94.94%   94.98%   +0.04%     
==========================================
  Files          10       11       +1     
  Lines         732      738       +6     
==========================================
+ Hits          695      701       +6     
  Misses         37       37              

Flag	Coverage Δ
unittests	94.98% <100.00%> (+0.04%)	⬆️

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Impacted Files	Coverage Δ
test/test_models_yolov5.py	100.00% <100.00%> (ø)
test/test_trainer.py	100.00% <100.00%> (ø)

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